git submodule -b ... of current HEAD fails

git submodule add -b $branch $repository

fails when HEAD already points to $branch in $repository.

Reported-by: Klaus Ethgen <Klaus@Ethgen.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -241,7 +241,7 @@ cmd_add()
 			# ash fails to wordsplit ${branch:+-b "$branch"...}
 			case "$branch" in
 			'') git checkout -f -q ;;
-			?*) git checkout -f -q -b "$branch" "origin/$branch" ;;
+			?*) git checkout -f -q -B "$branch" "origin/$branch" ;;
 			esac
 		) || die "Unable to checkout submodule '$path'"
 	fi