git-clone: Make sure the master branch exists before running cat on it.

Otherwise we get an error like this on stderr:

  cat: [...]/.git/refs/remotes/origin/master: No such file or directory

which makes it look like git-clone failed.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-clone.sh

@@ -355,7 +355,7 @@ then
 	# The name under $remote_top the remote HEAD seems to point at.
 	head_points_at=$(
 		(
-			echo "master"
+			test -f "$GIT_DIR/$remote_top/master" && echo "master"
 			cd "$GIT_DIR/$remote_top" &&
 			find . -type f -print | sed -e 's/^\.\///'
 		) | (