gitweb: Fix support for legacy gitweb config for snapshots

gitweb: Fix support for legacy gitweb config for snapshots

Earlier commit which cleaned up snapshot support and introduced
support for multiple snapshot formats changed the format of
$feature{'snapshot'}{'default'} (gitweb configuration) and
gitweb.snapshot configuration variable (repository configuration).
It supported old gitweb.snapshot values of 'gzip', 'bzip2' and 'zip'
and tried to support, but failed to do that, old values of
$feature{'snapshot'}{'default'}; at least those corresponding to
old gitweb.snapshot values of 'gzip', 'bzip2' and 'zip', i.e.
  ['x-gzip', 'gz', 'gzip']
  ['x-bzip2', 'bz2', 'bzip2']
  ['x-zip', 'zip', '']

This commit moves legacy configuration support out of feature_snapshot
subroutine to separate filter_snapshot_fmts subroutine. The
filter_snapshot_fmts is used on result on result of
gitweb_check_feature('snapshot').  This way feature_snapshot deals
_only_ with repository config.

As a byproduct you can now use 'gzip' and 'bzip2' as aliases to 'tgz'
and 'tbz2' also in $feature{'snapshot'}{'default'}, not only in
gitweb.snapshot.

While at it do some whitespace cleanup: use tabs for indent, but
spaces for align.

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