Dan Savilonis provided a large comprehensive change for improving our branch detection. Can you update your post-review from Git and see if the new code works for you?
Fix bug 1360 (diffs with parent diffs)
Review Request #1186 — Created Oct. 31, 2009 and discarded
Fix bug 1360 (diffs with parent diffs) In pure-git setups, post-review with parent diffs was broken because the parent diff wasn't being generated properly (it diffed master and the work tree instead of master and parent).
Set up a pure git repo (i.e. no git-svn or git-p4 or anything). In a local clone, have branches A and B such that B^ == A and A^ == master, and B modifies a line that A modifies (so `git diff A..B` applied to master doesn't apply cleanly). Run post-review with --parent=A and verify that the diff appears correctly on the site (previous behavior was to show an error because the patch didn't apply).