gitweb: parse project/action/hash_base:filename PATH_INFO

This patch enables gitweb to parse URLs with more information embedded
in PATH_INFO, reducing the need for CGI parameters. The typical gitweb
path is now $project/$action/$hash_base:$file_name or
$project/$action/$hash

This is mostly backwards compatible with the old-style gitweb paths,
$project/$branch[:$filename], except when it was used to access a branch
whose name matches a gitweb action.

Signed-off-by: Giuseppe Bilotta <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>