Make rebase-required messages clearer for users
Starts the process of improving our rebase-required message, as outlined in #327271. This work requires a FE engineer to complete, because it needs more than a UI text swap:
- Add a popover
(?)to the text.
- Popover wording: Rebase adds any changes from the target branch to the source branch. It's required before merging in projects using the fast-forward merge method. [Learn more]
- Learn more link destination: https://docs.gitlab.com/ee/topics/git/git_rebase.html
- Check documentation for needed updates in a separate MR. Our error message strings should be included in the text of relevant pages in our docs, so users running a "what do I do now" search get good info from GitLab, not a third party.
- Create follow-up issues as needed.