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

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