`MergeRequestDiff#save_git_content` can take a while to compute and save
the diffs, and it can hold a transaction in idle for a long time when it
attempts to update the latest `latest_merge_request_diff_id`.

To prevent this, we first generate the diff and only after this
completes do we update the diff ID.

Closes https://gitlab.com/gitlab-org/gitlab/issues/33650