1. Clone the repo to a temp directory.
2. Apply the MR as a patch.
3. Use `git commit -C` to copy the commit metadata from the HEAD commit
   of the source branch, and manually add the merging user as the
   committer.
4. Push back to a randomly-generated branch name.
5. Immediately delete that branch, so that it does not appear in the
   branch list, trigger CI builds, etc.
6. Return the SHA of the squashed commit we've just pushed to the
   MergeService.
7. In the MergeService, allow an arbitrary source to be used, and set
   that to either the source branch (if there is no squashing) or the
   squashed SHA.