Use `git worktree` for rebase operations

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1024 is using git worktree to squash, but we could also do this for rebase.

That means that we can remove the rebase_commit_sha column from the merge_requests table (added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/489), and probably simplify a bunch of steps in rebasing!