Merge branch 'cm/rebase-i'

Merge branch 'cm/rebase-i'

"rebase -i" is getting cleaned up and also enhanced.

* cm/rebase-i:
  doc/git-rebase: add documentation for fixup [-C|-c] options
  rebase -i: teach --autosquash to work with amend!
  t3437: test script for fixup [-C|-c] options in interactive rebase
  rebase -i: add fixup [-C | -c] command
  sequencer: use const variable for commit message comments
  sequencer: pass todo_item to do_pick_commit()
  rebase -i: comment out squash!/fixup! subjects from squash message
  sequencer: factor out code to append squash message
  rebase -i: only write fixup-message when it's needed