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

git-rebase--interactive.sh

@@ -574,6 +574,7 @@ EOF
 		has_action "$TODO" ||
 			die_abort "Nothing to do"
 
+		git update-ref ORIG_HEAD $HEAD
 		output git checkout $ONTO && do_rest
 		;;
 	esac