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: Charles McGarvey <redacted>
Signed-off-by: Junio C Hamano <redacted>