Rollback mutation of merge request on failed merge

What does this MR do?

The second call to the Gitaly UserMergeBranch RPC method can raise an exception. Previously, if this were to happen, the merge request's in_progress_merge_commit_sha attribute would be set to a commit SHA that didn't exist.

Repository#merge now uses a transaction block in order to rollback the setting of this attribute if any error occurs.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Closes #60720 (moved)