mq: drop the use of mq.queue.qparent in mq.queue.strip Same as in the previous changeset, rev is never `None`. We can just copy the two relevant lines in in `queue.strip`. This help having `queue.strip` independent from `queue`. One further step toward the extraction of `strip` in an independent extension. (As discussed in issue3824).

mq: drop the use of mq.queue.qparent in mq.strip In this case, rev is never `None`. We can just copy the two relevant lines in in `strip`. This help having `strip` independent from `queue` one further step toward its extraction in an independent extension. (As discussed in issue3824).

mq: document repo.mq.qparents The function is not very complex but writing this doc helped me to check if I got everything right.

mq: use the new checklocalchange in the strip command The strip command never use the `refresh` argument. So we can use the function we just extracted.

mq: extract checklocalchanges from `mq.queue` The core part of `checklocalchanges` is now mq independent. We can extract it in a standalone function to help the extraction of `strip` as discussed in issue3824. A `checklocalchanges` function stay in `mq.queue` with the part related to "refresh first" messages.

mq: extract checksubstate from the queue class This function does not need any of the the `mq.queue` method or attributes. It is indirectly used by the `strip` command. We are trying to extract this command in a standalone extension as discussed in issue issue3824.