Add action to update the MR source branch by fetching from elsewhere
The "Allow commits from members who can merge to the target branch." feature is not always enabled by the MR submitter, and it only allows maintainers that can merge to the target branch to push to the MR source branch rather than other developers. Another way to enable this workflow would be something like
Do: fetch other/repo branch
That way any developer can push a rewritten branch to their fork and then use it to update another dev's MR.