submodules: introduce check to see whether to touch a submodule

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

It is potentially expensive to check if a submodule needs an update,
because a common theme to interact with submodules is to spawn a child
process for each interaction.

So let's introduce a function that checks if a submodule needs
to be checked for an update before attempting the update.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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