gitweb: Fix export check in git_get_projects_list

gitweb: Fix export check in git_get_projects_list

When $filter was empty, the path passed to check_export_ok would
contain an extra '/', which some implementations of export_auth_hook
are sensitive to.

It makes more sense to fix this here than to handle the special case
in each implementation of export_auth_hook.

Signed-off-by: Devin Doucette <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>