pre-rebase hook update

This hook is what I have been using to manage topic branches in git.git,
but have not been updated to the Real Thing for a while.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

templates/hooks--pre-rebase

 #!/bin/sh
 #
-# Copyright (c) 2006 Junio C Hamano
+# Copyright (c) 2006, 2008 Junio C Hamano
 #
+# The "pre-rebase" hook is run just before "git-rebase" starts doing
+# its job, and can prevent the command from running by exiting with
+# non-zero status.
+#
+# The hook is called with the following parameters:
+#
+# $1 -- the upstream the series was forked from.
+# $2 -- the branch being rebased (or empty when rebasing the current branch).
+#
+# This sample shows how to prevent topic branches that are already
+# merged to 'next' branch from getting rebased, because allowing it
+# would result in rebasing already published history.
 
 publish=next
 basebranch="$1"
@@ -9,11 +21,12 @@ if test "$#" = 2
 then
 	topic="refs/heads/$2"
 else
-	topic=`git symbolic-ref HEAD`
+	topic=`git symbolic-ref HEAD` ||
+	exit 0 ;# we do not interrupt rebasing detached HEAD
 fi
 
-case "$basebranch,$topic" in
-master,refs/heads/??/*)
+case "$topic" in
+refs/heads/??/*)
 	;;
 *)
 	exit 0 ;# we do not interrupt others.
@@ -23,6 +36,12 @@ esac
 # Now we are dealing with a topic branch being rebased
 # on top of master.  Is it OK to rebase it?
 
+# Does the topic really exist?
+git show-ref -q "$topic" || {
+	echo >&2 "No such branch $topic"
+	exit 1
+}
+
 # Is topic fully merged to master?
 not_in_master=`git-rev-list --pretty=oneline ^master "$topic"`
 if test -z "$not_in_master"