MergeRequests::PushOptionsHandlerService has been updated to allow
creating and updating merge requests with the
`remove_source_branch` set using git push options.

To create a new merge request and set it to remove the source branch
when it is merged:

  git push -u origin -o merge_request.create \
    -o merge_request.remove_source_branch

To update an existing merge request and set it to remove the source
branch when it is merged:

  git push -u origin -o merge_request.remove_source_branch

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/64320