gitweb: Harden and improve $project_filter page title

Commit 19d2d23 (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: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>