[MERGE] Move RemoteRepository caching logic to CachedParentsProvider

[Martin Pool]

On 11 Dec 2008, Aaron Bentley <aaron at aaronbentley.com> wrote:

> > I'd prefer just a plain AssertionError to further bloating errors.py.  I
> > believe Martin has previously stated that we should try to avoid adding
> > errors that we never intend to catch.

I have, and I think it still makes sense.  If we're just going to show
it to a user as an error message, it doesn't really matter what class
would be raised.  If code would plausibly want to catch it, to know what
particular error was raised or what its parameters were, then it should
have a new class.

-- 
Martin