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).