Fix sample pre-commit hook

If the worktree happened to have a file called HEAD, "diff-index --cached HEAD"
would complain about the ambiguity between revision and path.  Avoid it by
using an explicit "--" for disambiguation.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-commit

@@ -13,7 +13,7 @@
 
 if git-rev-parse --verify HEAD 2>/dev/null
 then
-	git-diff-index -p -M --cached HEAD
+	git-diff-index -p -M --cached HEAD --
 else
 	# NEEDSWORK: we should produce a diff with an empty tree here
 	# if we want to do the same verification for the initial import.