submodule-config: add helper function to get 'fetch' config from .gitmodules

submodule-config: add helper function to get 'fetch' config from .gitmodules

Add a helper function to make it clearer that retrieving 'fetch'
configuration from the .gitmodules file is a special case supported
solely for backward compatibility purposes.

This change removes one direct use of 'config_from_gitmodules' in code
not strictly related to submodules, in the effort to communicate better
that .gitmodules is not to be used as a mechanism to store arbitrary
configuration in the repository that any command can retrieve.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>