[rfc] [patch] trivial patch to speed up bzr log

[Denys Duchier]

it is also necessary to clear the cached rev in weave when things change, and
protect it against side-effects.  revno=1507 contains a patch for this: in
particular it forcibly clears the cache in test_weave.Corruption.test_detection
because that test independently side-effects the weave without going through the
weave api.

Cheers,

--Denys