Ignore no-op changes in paranoid update hook

If the hook gets invoked with identical old and new ids there
is no change taking place.  We probably should not have been
called, but instead of failing silently allow the no-op.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/hooks/update-paranoid

@@ -259,6 +259,7 @@ deny "Refusing funny ref $ref" unless $ref =~ s,^refs/,,;
 deny "Bad old value $old" unless $old =~ /^[a-z0-9]{40}$/;
 deny "Bad new value $new" unless $new =~ /^[a-z0-9]{40}$/;
 deny "Cannot determine who you are." unless $this_user;
+grant "No change requested." if $old eq $new;
 
 $repository_name = File::Spec->rel2abs($git_dir);
 $repository_name =~ m,/([^/]+)(?:\.git|/\.git)$,;