When working with submodules it is easy to forget to push a
submodule to the server but pushing a super-project that
contains a commit for that submodule. The result is that the
superproject points at a submodule commit that is not available
on the server.

This adds the option --recurse-submodules=check to push. When
using this option git will check that all submodule commits that
are about to be pushed are present on a remote of the submodule.

To be able to use a combined diff, disabling a diff callback has
been removed from combined-diff.c.
Signed-off-by: NFredrik Gustafsson <iveqy@iveqy.com>
Mentored-by: NJens Lehmann <Jens.Lehmann@web.de>
Mentored-by: NHeiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>