Check out GH pull requests locally where the source/target branch had been deleted

## What does this MR do?

Check out GitHub pull requests where source/target branches that had been deleted locally rather than temporarily restoring them on GitHub using their References API. This helps us to not get rate limited, and allow us to import cross-repository pull requests (those from forks).

## What are the relevant issue numbers?

Fixes #15528

Fixes #17766

Fixes #19310

Fixes #19439

Fixes #19998

Fixes #20153

Fixes #20552

Fixes https://gitlab.com/gitlab-com/support-forum/issues/801

#20385

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [X] ~~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 !5630