diff: stop allowing diff to have submodules configured in .git/config

diff: stop allowing diff to have submodules configured in .git/config

Traditionally a submodule is comprised of a gitlink as well as a
corresponding entry in the .gitmodules file.  Diff doesn't follow this
paradigm as its config callback routine falls back to populating the
submodule-config if a config entry starts with 'submodule.'.

Remove this behavior in order to be consistent with how the
submodule-config is populated, via calling 'gitmodules_config()' or
'repo_read_gitmodules()'.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>