gitweb: git_get_project_config requires only $git_dir, not also $project

Fix overeager early return in git_get_project_config, introduced in 9be3614
(gitweb: Fix project-specific feature override behavior, 2010-03-01).  When
git_get_project_config is called from projects list page via
git_get_project_owner($path) etc., it is called with $git_dir defined (in
git_get_project_owner($path) etc.), but $project variable is not defined.
git_get_project_config doesn't use $project variable anyway.

Reported-by: Tobias Heinlein <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>