context: reuse changectx._copies() in all but workingctx This moves the dirstate-specific _copies() implementation from committablectx into workingctx where it should be (I think all dirstate-specific stuff should be moved into workingctx). The part of changectx._copies() that is for producing changeset-wide copy dicts from the filectxs is moved into basectx so it's reused by the other subclasses. The part of changectx._copies() that's about reading copy information from the changeset remains there. This fixes in-memory rebase (and makes `hg convert` able to write copies to changesets). Differential Revision: https://phab.mercurial-scm.org/D6219