Merge branch 'pw/advise-rebase-skip'
The mechanism to prevent "git commit" from making an empty commit
or amending during an interrupted cherry-pick was broken during the
rewrite of "git rebase" in C, which has been corrected.
* pw/advise-rebase-skip:
commit: give correct advice for empty commit during a rebase
commit: encapsulate determine_whence() for sequencer
commit: use enum value for multiple cherry-picks
sequencer: write CHERRY_PICK_HEAD for reword and edit
cherry-pick: check commit error messages
cherry-pick: add test for `--skip` advice in `git commit`
t3404: use test_cmp_rev