mq: extract strip function as its standalone extension (issue3824) Strip now lives in its own extension reminder: The extension is surprisingly called `strip`. The `mq` extension force the use of the strip extension when its enabled. This is both necessary for backward compatibility (people expect `mq` to comes with strip) and become some utility function used by `mq` are now in the strip extension.

strip: move the strip helper function for mq to strip The next patch finally move the command. No joke! (hey, this is for issue3824)

strip: move checklocalchanges from mq to strip One more step for issue3824.

strip: move checksubstate from mq to strip One more step for issue3824