Clean merge_jid whenever necessary on the merge process
MergeRequest#merge_jid should be cleaned up whenever we hit a known error on
MergeService#execute (or we finished processing). This way we can keep track if the MR is really "ongoing" or just "stuck" (stuck MRs are already being handled by
Currently, if a an user clicks
Merge and the MR is not mergeable, or no source is found, we keep stuck on
merge ongoing state.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- API support added
- Tests added for this feature/bug
- Has been reviewed by Backend
Conform by the [merge request performance guides](https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
Conform by the style guides
- Squashed related commits together
What are the relevant issue numbers?
Closes #38476 (closed)