Small patch for CHK branch to fix some tests

[Andrew Bennetts]

Robert Collins wrote:
> On Tue, 2008-11-04 at 13:40 +1100, Andrew Bennetts wrote:
> > Hi Robert,
> > 
> > I haven't run the full suite, but this patch appears to help at least
> > test_upgrade_preserves_signatures, and makes one variant of test_fetch_knit3 get
> > a little further along too.  I bumped into these yesterday while playing with
> > usertest on your branch.
> 
> Setting the children directly is not permitted in the new code; allowing
> that would be more than a little complex. So I'm not inclined to merge
> that part of the patch without more understanding of what is failing and
> why.

Something was trying to assign to it.  I'll repeat and give you a more specific
exception in a little while.  (I foolishly let the exception disappear off my
scrollback.)

> The other change would seem to be a no-op, could you expand on why its
> needed?

CHKInventory doesn't provide ._byid (and neither does the CommonInventory base
class).  But it does provide __iter__, which is just as good here, and using
that was easier than implementing a _byid for CHKInventory.

-Andrew.