gitweb: retrieve snapshot format from PATH_INFO

gitweb: retrieve snapshot format from PATH_INFO

We parse requests for $project/snapshot/$head.$sfx as equivalent to
$project/snapshot/$head?sf=$sfx, where $sfx is any of the known
(although not necessarily supported) snapshot formats (or its default
suffix).

The filename for the resulting package preserves the requested
extensions (so asking for a .tgz gives a .tgz, and asking for a .tar.gz
gives a .tar.gz), although for obvious reasons it doesn't preserve the
basename (git/snapshot/next.tgz returns a file names git-next.tgz).

This introduces a potential case for ambiguity if a project has a head
that ends with a snapshot-like suffix (.zip, .tgz, .tar.gz, etc) and the
sf CGI parameter is not present; however, gitweb only produces URLs with
the sf parameter currently, so this is only a potential issue for
hand-coded URLs for extremely unusual project.

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