submodule-config: add helper to get 'update-clone' config from .gitmodules

submodule-config: add helper to get 'update-clone' config from .gitmodules

Add a helper function to make it clearer that retrieving 'update-clone'
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' for
options not strictly related to submodules: "submodule.fetchjobs" does
not describe a property of a submodule, but a behavior of other commands
when dealing with submodules, so it does not really belong to the
.gitmodules file.

This is 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>