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.

mq: simplifies the refresh hint in checklocalchanges The `checklocalchanges` function in the `mq.queue` class takes a `refresh` argument that changes the error message of raised exception. When refresh is `True` the exception message is "local changes found, refresh first" otherwise, the message is just "local changes found". This changeset is the first of a series that extract `strip` into a standalone extension (as discussed in issue3824). This `checklocalchanges` function is indirectly used by the strip command. But in a standalone strip extension the concept of "refresh first" has no sense. In practice, When used in the context of the strip commands `refresh`'s value is always `False`. So my final goal is a be able to extract the `checklocalchanges` logic in a standalone extension but to keep the part related to "refresh first" in the mq extension. However the refresh handling is deeply entangled into the `checklocalchanges` code. It is handled as low a possible at the point we raise the exception. So we moves handling of refresh upper in the `checklocalchanges` code. This will allow the extraction of a simple version in the strip extension while mq can still inject its logic when needed. Two helper functions `localchangesfound` and `localchangedsubreposfound` died in the process they are replaced by simple raise lines.