Fix resolving conflicts on forks

## What does this MR do?

When we resolve conflicts, we create a merge commit in the source branch with parents `[source_branch_head, target_branch_head]`. But when the MR is from a fork, `target_branch_head` might not exist in the source repo at all, so we need to fetch it if it isn't there. We can do this locally so it should be fast.

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

The `TestEnv` changes are needed to reset the branch refs if we're reusing a git directory locally - otherwise, there might not be conflicts!

## Why was this MR needed?

It's a bug in a new feature!

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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)

## What are the relevant issue numbers?

Closes #21459.

See merge request !6082