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 <brlink@debian.org>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -3979,9 +3979,11 @@ sub git_footer_html {
 		}
 
 	} else {
-		print $cgi->a({-href => href(project=>undef, action=>"opml"),
+		print $cgi->a({-href => href(project=>undef, action=>"opml",
+		                             project_filter => $project_filter),
 		              -class => $feed_class}, "OPML") . " ";
-		print $cgi->a({-href => href(project=>undef, action=>"project_index"),
+		print $cgi->a({-href => href(project=>undef, action=>"project_index",
+		                             project_filter => $project_filter),
 		              -class => $feed_class}, "TXT") . "\n";
 	}
 	print "</div>\n"; # class="page_footer"