gitweb: Make project specific override for 'grep' feature work

The 'grep' feature was marked in the comments as having project
specific config, but it lacked 'sub' key required for it to work.

Kind-of-Noticed-by: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -232,6 +232,7 @@ our %feature = (
 	# $feature{'grep'}{'override'} = 1;
 	# and in project config gitweb.grep = 0|1;
 	'grep' => {
+		'sub' => \&feature_grep,
 		'override' => 0,
 		'default' => [1]},