t/gitweb-lib.sh: set $REQUEST_URI

t/gitweb-lib.sh: set $REQUEST_URI

In a real webserver's CGI call, gitweb.cgi would typically see
$REQUEST_URI set. This variable does impact how we display our URL in
the resulting page, so let's try to make our test as realistic as
possible (we can just use the $PATH_INFO our caller passed in, if any).

This doesn't change the outcome of any tests, but it will help us add
some new tests in a future patch.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>