Merge branch 'hu/cherry-pick-previous-branch'

Just like "git checkout -" knows to check out and "git merge -"
knows to merge the branch you were previously on, "git cherry-pick"
now understands "git cherry-pick -" to pick from the previous
branch.

* hu/cherry-pick-previous-branch:
cherry-pick: allow "-" as abbreviation of '@{-1}'

diff --cc builtin/revert.c
Simple merge

diff --cc t/t3501-revert-cherry-pick.sh
Simple merge