Merge branch 'ml/log-merge-with-cherry-pick-and-other-pseudo-heads'

"git log --merge" learned to pay attention to CHERRY_PICK_HEAD and other kinds of *_HEAD pseudorefs. * ml/log-merge-with-cherry-pick-and-other-pseudo-heads: revision: implement `git log --merge` also for rebase/cherry-pick/revert revision: ensure MERGE_HEAD is a ref in prepare_show_merge