Don't load missing ACL files in paranoid update hook

If a user or group ACL file does not exist in the current tip
revision of the acl repository we will get an error from cat-file
when we ask for that blob as it cannot be resolved.  A quick look
at the history by rev-list can tell us if there is a path there
or not.

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

contrib/hooks/update-paranoid

@@ -136,6 +136,7 @@ sub parse_config ($$$$) {
 	local $ENV{GIT_DIR} = shift;
 	my $br = shift;
 	my $fn = shift;
+	return unless git_value('rev-list','--max-count=1',$br,'--',$fn);
 	info "Loading $br:$fn";
 	open(I,'-|','git','cat-file','blob',"$br:$fn");
 	my $section = '';