gitweb: Harden and improve $project_filter page title

Commit 19d2d239 (gitweb: add project_filter to limit project list
to a subdirectory, 2012-01-30) added also support for displaying
$project_filter, if present, in page title.

Unfortunately it forgot to treat $project_filter as path, and escape
it using esc_path(), like it is done for $filename.

Also, it was not obvious that "$site_name - $project_filter" is about
project filtering: use "$site_name - projects in '$project_filter'".
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -3742,7 +3742,7 @@ sub get_page_title {
 
 	unless (defined $project) {
 		if (defined $project_filter) {
-			$title .= " - " . to_utf8($project_filter);
+			$title .= " - projects in '" . esc_path($project_filter) . "'";
 		}
 		return $title;
 	}