Merge branch 'jc/revert-show-parent-info'

"git revert" learns "--reference" option to use more human-readable
reference to the commit it reverts in the message template it
prepares for the user.

* jc/revert-show-parent-info:
  revert: --reference should apply only to 'revert', not 'cherry-pick'
  revert: optionally refer to commit in the "reference" format

diff --cc sequencer.c
Simple merge

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