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