Filing this as a defect because it will protect us from
the nasty consequences of certain kinds of working copy
corruption (for example, the bug Ben encountered once
whereby his working file became his text-base, without
any intervening commit; a bug we haven't even a hint of
a reproduction recipe for, but which would be at least
caught by a checksum, allowing the user to re-checkout
their working copy).
So: make sure the entry records a checksum for text-base
at all times. Deciding *when* to use that checksum is
a slightly more complex question: I think we probably want
to do it whenever we detect a local mod on that file, which
includes whenever we commit it.
Blocker