Edit commit messages when merging merge request
- When merging a merge request, provide a UX flow to edit commit messages as those commits are being applied to the target branch.
- This will allow the target branch maintainer to ensure commit messages follow any desired format, has the right information, correct typos, and just ensure a desired commit history.
- This should appear in the merge request widget during the merge UX flow.
- This would be a follow up / enhancement of https://gitlab.com/gitlab-org/gitlab-ee/issues/1510.