gitweb: add project_filter to limit project list to a subdirectory

This commit changes the project listing views (project_list,
project_index and opml) to limit the output to only projects in a
subdirectory if the new optional parameter ?pf=directory name is
used.

The implementation of the filter reuses the implementation used for
the 'forks' action (i.e. listing all projects within that directory
from the projects list file (GITWEB_LIST) or only projects in the
given subdirectory of the project root directory without a projects
list file).

Reusing $project instead of adding a new parameter would have been
nicer from a UI point-of-view (including PATH_INFO support) but
would complicate the $project validating code that is currently
being used to ensure nothing is exported that should not be viewable.

Signed-off-by: Bernhard R. Link <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>