Stefan Beller [off-list ref] writes:

Integrate updating the submodules into git checkout, with the same
safety promises that git-checkout has, i.e. not throw away data unless
asked to. This is done by first checking if the submodule is at the same
sha1 as it is recorded in the superproject.

I've so far read only the first half of this series (i.e. the
preparatory part) and haven't reached the place where it starts to
become really interesting, but what I've seen so far looked very
sensible.  I also find that comments given by others so far all
raise good points.