doc/git-fsck: change the way for getting heads' SHA1s

The straightforward way with using 'cat .git/refs/heads/*' doesn't work
with packed refs as well as branches of the form topic/topic1. So let's
use git-for-each-ref for getting the heads' SHA1s in this example.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fsck.txt

@@ -79,7 +79,8 @@ that aren't readable from any of the specified head nodes.
 
 So for example
 
-	git fsck --unreachable HEAD $(cat .git/refs/heads/*)
+	git fsck --unreachable HEAD \
+		$(git for-each-ref --format="%(objectname)" refs/heads)
 
 will do quite a _lot_ of verification on the tree. There are a few
 extra validity tests to be added (make sure that tree objects are