Properly abort a merge when merge conflicts occur
If somehow a user attempted to accept a merge request that had conflicts (e.g. the "Accept Merge Request" button or the MR itself was not updated),
MergeService did not properly detect that a conflict
occurred. It would assume that the MR went through without any issues and close the MR as though everything was fine. This could cause data loss if the source branch were removed.
Closes #20425 (closed)