Ensure only IDs ending in .git perform git actions

## What does this MR do?

Rails's routing is pretty strange. Previously, `GET /namespace/project/info/refs` would go to the Git HTTP controller (if the redirect for that case was taken out).

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

The specs fail if the redirect is moved to above the Git HTTP routes, removed altogether, or the Git HTTP constraints are changed. But there might still be missing cases.

## Why was this MR needed?

The master build and HTTP cloning were both broken.

## What are the relevant issue numbers?

Closes #18376.

## Screenshots (if relevant)

Nope.

## Does this MR meet the acceptance criteria?

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

See merge request !4558