Merge branch 'mz/rebase-no-mbox'

Merge branch 'mz/rebase-no-mbox'

Teach "am --rebasing" codepath to grab authorship, log message and
the patch text directly out of existing commits.  This will help
rebasing commits that have confusing "diff" output in their log
messages.

* mz/rebase-no-mbox:
  am: don't call mailinfo if $rebasing
  am --rebasing: get patch body from commit, not from mailbox
  rebase --root: print usage on too many args
  rebase: don't source git-sh-setup twice