phases: call filterunknown() in readroots() One less function manipulating localrepo state.

repair: no need to call filterunknown() in strip() Calling strip() will eventually trigger localrepo.destroyed() which will invalidate _parseroots. It will call filterunknown() upon reload. Changes to test-keyword.t are related to commit --debug running after either qpop or rollback.

test-phases: test changing null revision phase The behaviour is correct but for bad reasons: the repo.set() call in phase command fails for '-1'. It should be rejected explicitely by phase boundary commands, sadly this is hard to do because phase changes are not applied atomically.