Fix duplicate head_commit_shas issues with merge_request_diffs
This MR fixes the issues where commits were pushed in quick succession which could cause a merge_request_diff to be generated with duplicated commit SHA occasionally. Since we know the head SHA from the push, we should make sure we only create a merge_request_diff for with the given SHA.
Related to #342009 (closed)
Edited by Sincheol (David) Kim