Add S_ISGITLINK subroutine and S_IFGITLINK, S_IFINVALID constants.
Add support for "commit" (submodule) entries in the tree object to
mode_str ('m---------', following cgit), file_type and file_type_long
('submodule') subroutines.

There is only link to the history of submodule entry in the
supermodule (current repository) for now, because gitweb doesn't know
where to search for submodule repository objects.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>