Add support for 'show-sizes' feature to show (in separate column,
between mode and filename) the size of blobs (files) in the 'tree'
view.  It passes '-l' option to "git ls-tree" invocation.

For the 'tree' and 'commit' (submodule) entries, '-' is shown in place
of size; for generated '..' "up directory" entry nothing is shown.

The 'show-sizes' feature is enabled by default.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>