Changes reach a release branch as backports — cherry-picks with new SHAs — so
the original quattro merge commit is never an ancestor of a patch branch and
the ancestry filter let already-applied PRs through. Candidates are now also
matched against the branch's own commit messages since it left quattro:
'backport of #N' / squash '(#N)' references and 'cherry picked from commit
<sha>' trailers (which pick -x itself writes). Explicitly named PRs/commits
that are already on the branch are skipped with a note instead of re-picked.
Verified against the live v4-0-2 branch: the five backported PRs it carries
filter out; un-backported ones are still offered.
Ryan Hughes
97519fb0f1