gitweb: fix problem causing erroneous project list

The bug is manifest when running gitweb in a persistent process (e.g.
FastCGI, PSGI), and it's easy to reproduce.  If a gitweb request
includes the searchtext parameter (i.e. s), subsequent requests using
the project_list action--which is the default action--and without
a searchtext parameter will be filtered by the searchtext value of the
first request.  This is because the value of the $search_regexp global
(the value of which is based on the searchtext parameter) is currently
being persisted between requests.

Instead, clear $search_regexp before dispatching each request.
Signed-off-by: NCharles McGarvey <chazmcgarvey@brokenzipper.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -1086,7 +1086,7 @@ sub evaluate_and_validate_params {
 	our $search_use_regexp = $input_params{'search_use_regexp'};
 
 	our $searchtext = $input_params{'searchtext'};
-	our $search_regexp;
+	our $search_regexp = undef;
 	if (defined $searchtext) {
 		if (length($searchtext) < 2) {
 			die_error(403, "At least two characters are required for search parameter");