[RFC] Cache optimizations

Does anyone know of a reason not to refresh each entry in the cache as
it's checked out? It seems to me like this would eliminate the need to
call update-cache --refresh after checkout-cache and it would update only
the entries for files which are actually changed in the working directory.

Is it just that the code to lock and write the cache is
(currently) inconvenient to add to different programs?

	-Daniel
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