Close merge requests and don't refresh merge request when a source branch is deleted

From https://gitlab.com/gitlab-org/gitlab-ce/issues/32812#note_39580785:

Let's say you have a open merge request with source_branch to master.

Right now if you delete source_branch and then push to master, we attempt to reload master. Since the ref for source_branch no longer exists, we end up updating the merge request with an empty diff (for example: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13003). We should:

1. Avoid updating the merge request if the source SHA is nil
2. Close open merge requests with that branch

/cc: @smcgivern, @vsizov, @victorwu