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 StuckMergeJobsWorker).
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.
Backport https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3027
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
-
Documentation created/updated - API support added
- Tests added for this feature/bug
- Review
- 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)