update submodules: add a config option to determine if submodules are updated

update submodules: add a config option to determine if submodules are updated

In later patches we introduce the options and flag for commands
that modify the working directory, e.g. git-checkout.

Have a central place to store such settings whether we want to update
a submodule.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>