When making a release from master, the range since the last minor
release includes all the commits which were cherry-picked to the
previous stable branch and already published in its point releases,
so their changelog lines had to be deleted by hand.
make_changelog.py now finds the previous stable branch (eg
v1.74-stable) from the version being released and skips any commits
released there, detected via cherry-pick -x trailers and git cherry
patch equivalence. Skipped commits are listed on stderr for review.
Releases made from a stable branch are unaffected.
Nick Craig-Wood
ed983c952d