Fix markdown rendering for label references

Fixes #14424, #19753, #19760

See merge request !5224

Upgrade Rails from 4.2.6 to 4.2.7.

Primarily just bug fixes.

See merge request !5236

This reverts commit 9ca633eb, reversing
changes made to fb229bbf.

Primarily just bug fixes.

Changelog: https://github.com/rails/rails/compare/v4.2.6...v4.2.7

Actually render old and new sections of parallel diff next to each other

See merge request !5173

Dropdown loading time preformance fix

## What does this MR do?
Optimizes the performance of the dropdown load time by just sending the required data to load the dropdown instead of the full object

This MR aims to fix #17474 

See merge request !5113

Keeps issue number when importing from Gitlab.com

## What does this MR do?

Keeps issue number when importing from `Gitlab.com`

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

With these changes we doesn't loose the issue references when importing from `GitLab.com`.

## What are the relevant issue numbers?

Closes #15235 

/cc @cpm @jweerdt

See merge request !5193

With these changes we don’t lost the issue references when importing
from `GitLab.com`.

Fix Knapsack report generation for Spinach

## What does this MR do?
Makes Knapsack auto-balacing work for Spinach tests

## Why was this MR needed?
We used wrong Knapsack adapter, thus making spinach to run split, but not auto-balanced tests for spinach.


See merge request !5230

Allow developers to merge into a protected branch without having push access

## What does this MR do?

Adds a "Developers can merge" checkbox to protected branches much like the "Developers can push" checkbox. When the checkbox is enabled, a developer can merge MRs into that protected branch from the Web UI and from the command-line (any push that is entirely composed of merge commits is allowed).

## Are there points in the code the reviewer needs to double check?

- This MR refactors the `GitAccess` module, moving parts of it to `UserAccess` and the new `ChangeAccessCheck`.
- This MR refactors `GitAccessSpec`, which generates a "matrix" of tests.
- The main logic "developers can merge" should be straightforward enough.
- The commits are fairly atomic, and the commit messages are descriptive regarding the motivations behind every change.

## Why was this MR needed?

A significant portion of this feature was implemented in !4220 (thanks, @mvestergaard!) ; I'm wrapping it up.

## What are the relevant issue numbers?

#18193 
Closes #967 

## Screenshots

![1](/uploads/c636e88ba38628211754e7cf122b0dc4/1.png)
![2](/uploads/5ed1e7917e2f36853a479faa565b022a/2.png)
![3](/uploads/0d202ba42e8dc6aade7bc6ac8db41ee6/3.png)

## TODO

- [ ]  #18193 !4892 Add "developers can merge" as an option for protected branches
    - [x]  Review existing code
    - [x]  Fix build
    - [x]  Implementation / refactoring
        - [x]  Clean up `GitAccess`
        - [x]  Clean up `protected_branches.js.coffee`
        - [x]  Figure out authorization issue
            - If we try to merge code into a protected branch for a user who doesn't have access to that branch, an auth check will fail
            - We need to get around this, somehow
            - [x]  Try detecting merge commits and allowing those
        - [x]  A push with regular commits _and_ merge commits should fail
            - [x]  Figure out a solution
            - [x]  Extensive tests for `MergeCommitCheck`
    - [x]  Add tests
        - [x]  Untested parts of original MR
    - [x]  Improve the checks in `/allowed`
        -  @dzaporozhets's proposal:
            - commits in push == commits in merge request
            - branch to push == target branch of merge request
            - merge request has required amount of approves (ee only)
            - merge commit in push == merge commit we created when merged via UI
            - save merge commit sha in database and compare with `newrev`
        - my proposal
            - /allowed finds all open merge requests with the appropriate target branch
            - For each MR, compare the commit SHAs in the MR to the commit SHAs in the change set
            - If we have a match, compare the diff of the matching MR to the diff of the change set
            - If we still have a match, the merge is legit
        - [x]  Wait for replies on my proposal
        - [x]  Pick a strategy
        - [x]  Implementation
            - [x]  Save `in_progress_merge_commit_sha`
            - [x]  Check `in_progress_merge_commit_sha`
            - [x]  Clear `in_progress_merge_commit_sha`
        - [x]  Test / refactor
    - [x]  Merge conflicts
    - [x]  Verify workflows
        - [x]  Developer with 'developer can merge' on:
            - [x]  Can merge an MR from the Web UI
            - [x]  Error message for conflicts in the Web UI
            - [x]  Cannot merge an MR from the command line (HTTP)
            - [x]  Cannot merge an MR from the command line (SSH)
            - [x]  Cannot modify the branch otherwise
        - [x]  Developer with 'developer can merge' off:
            - [x]  Cannot merge an MR from the Web UI
            - [x]  Error message for conflicts in the Web UI
            - [x]  Cannot merge an MR from the command line (HTTP)
            - [x]  Cannot merge an MR from the command line (SSH)
            - [x]  Cannot modify the branch otherwise
        - [x]  New projects created could have have "Developers can merge" turned on automatically for the default branch
    - [x]  CHANGELOG
    - [x]  Fix build
    - [x]  Wait for [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/42624e3d53754064186d4ae9048e310d1d3eed0b/builds) to pass
    - [x]  Screenshots
    - [x]  Assign to endboss
    - [x]  Respond to @dbalexandre's comments
        - [x]  Duplicated line, this is equals to line 26.
        - [x]  We aren't using any of these helpers in this migration, we can remove the include.
        - [x]  What do you think to add a default value for this column to avoid the Three-state Boolean Problem?
        - [x]  group all checks under Gitlab::Checks
        - [x]  You have a default value for developers_can_merge column, but your migration doesn't add it.
        - [x]  What do you think to rename Partially protected to anything else?
    - [x]  Fix conflicts
    - [x]  Make sure [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/b1cfd42f20a78fd7f844288954e97cff32962e20/builds) passes
    - [ ]  Wait for merge

See merge request !4892

ObjectRenderer doesn't crash when no objects to cache with Rails.cache.read_multi

## What does this MR do?

Avoid calls to Rails.cache.read_multi without cache keys so it doesn't raise an exception

## What are the relevant issue numbers?

Closes #19766

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added. I considered is not needed is a fix over a RC
- ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~[ ] API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5229

1. Fix typos, minor styling errors.

2. Use single quotes rather than double quotes in `user_access_spec`.

3. Test formatting.

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4892#note_12892160
- This is more consistent.

1. If they are a developer with "Developers can Merge" switched on.

- Only send a param for the currently changed checkbox.
- Have the controller use strong parameters correctly, so that the PATCH
  works as expected.

1. Don't use case statements for dispatch anymore. This leads to a lot
   of duplication, and makes the logic harder to follow.

2. Remove duplicated logic.

    - For example, the `can_push_to_branch?` exists, but we also have a
      different way of checking the same condition within `change_access_check`.

    - This kind of duplication is removed, and the `can_push_to_branch?`
      method is used in both places.

3. Move checks returning true/false to `UserAccess`.

    - All public methods in `GitAccess` now return an instance of
      `GitAccessStatus`. Previously, some methods would return
      true/false as well, which was confusing.

    - It makes sense for these kinds of checks to be at the level of a
      user, so the `UserAccess` class was repurposed for this. The prior
      `UserAccess.allowed?` classmethod is converted into an instance
      method.

    - All external uses of these checks have been migrated to use the
      `UserAccess` class

4. Move the "change_access_check" into a separate class.

    - Create the `GitAccess::ChangeAccessCheck` class to run these
      checks, which are quite substantial.

    - `ChangeAccessCheck` returns an instance of `GitAccessStatus` as
      well.

5. Break out the boolean logic in `ChangeAccessCheck` into `if/else`
   chains - this seems more readable.

6. I can understand that this might look like overkill for !4892, but I
   think this is a good opportunity to clean it up.

    - http://martinfowler.com/bliki/OpportunisticRefactoring.html

1. When a merge request is being merged, save the merge commit SHA in
   the `in_progress_merge_commit_sha` database column.

2. The `pre-receive` hook looks for any locked (in progress) merge
   request with `in_progress_merge_commit_sha` matching the `newrev` it
   is passed.

3. If it finds a matching MR, the merge is legitimate.

4. Update `git_access_spec` to test the behaviour we added here. Also
   refactored this spec a bit to make it easier to add more contexts / conditions.

- Cherry-picked from `mvestergaard:branch-protection-dev-merge`
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4220

Render inline diffs for multiple changed lines following eachother

Before:

![Screen_Shot_2016-07-11_at_00.08.27](/uploads/b14664211e0f5cef6e77a78eadfcbcdf/Screen_Shot_2016-07-11_at_00.08.27.png)

After:

![Screen_Shot_2016-07-11_at_00.07.34](/uploads/567be631869a4867a2edf6ff7eda6369/Screen_Shot_2016-07-11_at_00.07.34.png)

See merge request !5174