Check what happens if idiots attach PRs in later branches first #1311
Description
In (r1, b1), have PR 1, merge, forward port to branches 2, 3.
Then in r2 create a linked pr in branch 3, then a bit later create a linked Pr in branch 2.
Currently the mergebot gets really snippy as it repeatedly tries to "complete" the branch 3 batch with the PR from branch 2. It should probably see that it does not need the forward port, and treat the branch 3 PR as the explicitly pushed forward port of branch 2.