tests: add tests for server-side linknode adjustment with wireprotov2 The current implementation of linknode serving in wireprotov2 simply serves up the linkrev/linknode as stored: it doesn't attempt to adjust the linknode to what the receiver is aware of. This can result in the client seeing a linknode referencing a changeset that is unknown to it. This commit adds test coverage of that scenario. The tests in test-wireproto-command-filesdata.t demonstrate two failures. First, the linknode refers to a changeset not in the available set. Second, the server doesn't send a file revision that it should have (because of linkrev filtering). The test in test-wireproto-exchange.t demonstrates that the lack of a file revision results in a corrupted repository on the client. Differential Revision: https://phab.mercurial-scm.org/D5404