verify: move cross-checking of changeset/manifest out of _crosscheckfiles() Reasons: * _crosscheckfiles(), as the name suggests, is about checking that the set of files files mentioned in changesets match the set of files mentioned in the manifests. * The "checking" in _crosscheckfiles() looked rather strange, as it just emitted an error for *every* entry in mflinkrevs. The reason was that these were the entries remaining after the call to _verifymanifest(). Moving all the processing of mflinkrevs into _verifymanifest() makes it much clearer that it's the remaining entries that are a problem. Functional change: progress is no longer reported for "crosschecking" of missing manifest entries. Since the crosschecking phase takes a tiny fraction of the verification, I don't think this is a problem. Also, any reports of "changeset refers to unknown manifest" will now come before "crosschecking files in changesets and manifests".