When creating a cross project merge request without changes. The MR
widget shows the option to create a new file. It would however link to
the wrong project.
When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.
This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)
Closes#40301
