config: add config_from_gitmodules

config: add config_from_gitmodules

Add 'config_from_gitmodules()' function which can be used by 'fetch' and
'update_clone' in order to maintain backwards compatibility with
configuration being stored in .gitmodules' since a future patch will
remove reading these values in the submodule-config.

This function should not be used anywhere other than in 'fetch' and
'update_clone'.

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