context: add workingfilectx.markcopied With in-memory merge, copy information needs to be stored in-memory, not in the dirstate. To make this transition easy, move the existing dirstate-based approach to workingfilectx; that way, other implementations can choose to store it somewhere else. Differential Revision: https://phab.mercurial-scm.org/D1106

merge: don't use workers in in-memory mode The worker processes can't share memory, so workers should not be used. Differential Revision: https://phab.mercurial-scm.org/D1105