check doesn't look for duplicate paths in an inventory

[Robert Collins]

I disabled this with the check performance overhaul; I've never seen it
fire, and we have lots of safeguards against it.

Its worth putting in, and I'd do it per-format, because chk should be
able to do it very efficiently.

That said, chk physically can't do it because it would require a
duplicate key in the CHK id,basename dict, and doing that just replaces
the key. Possibly we need a cross-check for the size of the two maps,
which would detect this being done.

 affects bzr