Merge branch 'js/advise-rebase-skip' into jch

Merge branch 'js/advise-rebase-skip' into jch

The logic used in "git commit" to give hints and errors depending
on what operation was in progress learned to distinguish rebase and
cherry-pick better.

* js/advise-rebase-skip:
  commit: give correct advice for empty commit during a rebase
  sequencer: export the function to get the path of `.git/rebase-merge/`
  cherry-pick: add test for `--skip` advice in `git commit`