gitweb: use_pathinfo filenames start with /

gitweb: use_pathinfo filenames start with /

Generate PATH_INFO URLs in the form project/action/hash_base:/filename
rather than project/action/hash_base:filename (the latter form is still
accepted in input).

This minimal change allows relative navigation to work properly when
viewing HTML files in raw ('blob_plain') mode.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>