Erroneous fast forward rebase error being displayed when merge trains are turned on and should not result in the issue.

When using merge trains with semi-linear history and the target branch of an MR changes after a pipeline has already run, an error box is displayed saying that "Fast forward merge is not possible. Please rebase".

Previously (back in the fall), the button changed from "Merge" to "Set to auto-merge", but continued to work and didn't report an error.

As of this month, the error message is new, it indicates that the merge is blocked AND pressing the button allows it to continue normally (unless there is an actual conflict between the branches).

This has resulted in confusion and people rebasing when it was not necessary to do so.

Note: This is a relatively new occurrence, and was working previous to this month or so.

Likely related to: #573986 (closed)

cc: @marc_shaw