gitweb: snapshot cleanups & support for offering multiple formats

gitweb: snapshot cleanups & support for offering multiple formats

- Centralize knowledge about snapshot formats (mime types, extensions,
  commands) in %known_snapshot_formats and improve how some of that
  information is specified.  In particular, zip files are no longer a
  special case.

- Add support for offering multiple snapshot formats to the user so
  that he/she can download a snapshot in the format he/she prefers.
  The site-wide or project configuration now gives a list of formats
  to offer, and if more than one format is offered, the "_snapshot_"
  link becomes something like "snapshot (_tar.bz2_ _zip_)".

- If only one format is offered, a tooltip on the "_snapshot_" link
  tells the user what it is.

- Fix out-of-date "tarball" -> "archive" in comment.

Alert for gitweb site administrators: This patch changes the format of
$feature{'snapshot'}{'default'} in gitweb_config.perl from a list of
three pieces of information about a single format to a list of one or
more formats you wish to offer from the set ('tgz', 'tbz2', 'zip').
Update your gitweb_config.perl appropriately.  There was taken care
for old-style gitweb configuration to work as it used to, but this
backward compatibility works only for the values which correspond to
gitweb.snapshot values of 'gzip', 'bzip2' and 'zip', i.e.
  ['x-gzip', 'gz', 'gzip']
  ['x-bzip2', 'bz2', 'bzip2']
  ['x-zip', 'zip', '']

The preferred names for gitweb.snapshot in repository configuration
have also changed from 'gzip' and 'bzip2' to 'tgz' and 'tbz2', but
the old names are still recognized for compatibility.

Signed-off-by: Matt McCutchen <hashproduct@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>