gitweb: fix regression when filtering out forks

This fixes a condition in filter_forks_from_projects_list that failed if
process directory was different from project root: in such case, the subroutine
was a no-op and forks were not detected.

Signed-off-by: Julien Muchembled <redacted>
Tested-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>