gitweb: limit links to alternate forms of project_list to active project_filter

gitweb: limit links to alternate forms of project_list to active project_filter

If project_list action is given a project_filter argument, pass that to
TXT and OPML formats.

This way [OPML] and [TXT] links provide the same list of projects as
the projects_list page they are linked from.

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