Merge branch 'jc/rebase-orig-head'

* jc/rebase-orig-head:
  Documentation: mention ORIG_HEAD in am, merge, and rebase
  Teach "am" and "rebase" to mark the original position with ORIG_HEAD

diff --cc Documentation/git-am.txt
Simple merge

diff --cc git-am.sh
Simple merge

diff --cc git-rebase--interactive.sh
Simple merge