git-rebase: allow rebasing a detached HEAD.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-rebase.sh

@@ -275,8 +275,12 @@ case "$#" in
 	git-checkout "$2" || usage
 	;;
 *)
-	branch_name=`git symbolic-ref HEAD` || die "No current branch"
-	branch_name=`expr "z$branch_name" : 'zrefs/heads/\(.*\)'`
+	if branch_name=`git symbolic-ref -q HEAD`
+	then
+		branch_name=`expr "z$branch_name" : 'zrefs/heads/\(.*\)'`
+	else
+		branch_name=HEAD ;# detached
+	fi
 	;;
 esac
 branch=$(git-rev-parse --verify "${branch_name}^0") || exit