gitweb: parse parent..current syntax from PATH_INFO

gitweb: parse parent..current syntax from PATH_INFO

This patch makes it possible to use an URL such as
project/action/somebranch..otherbranch:/filename to get a diff between
different version of a file. Paths like
project/action/somebranch:/somefile..otherbranch:/otherfile are parsed
as well.

All '*diff' actions and in general actions that use $hash_parent[_base]
and $file_parent (e.g. 'shortlog') can now get all of their parameters
from PATH_INFO

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>