pre-commit.sample: don't print incidental SHA1

Make the sample pre-commit hook script discard
all git-rev-parse output, not just stderr.
Otherwise, it would print an SHA1.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-commit.sample

@@ -7,7 +7,7 @@
 #
 # To enable this hook, rename this file to "pre-commit".
 
-if git-rev-parse --verify HEAD 2>/dev/null
+if git-rev-parse --verify HEAD >/dev/null 2>&1
 then
 	against=HEAD
 else