largefiles: avoid dynamically subclassing context instances E.g. copies.pathcopies() compares context objects for equality and basectx.__eq__ compares the type. But since largefiles was dynamically creating classes, they would all be unequal. That led pathcopies(), after some changes I made, to not short-circuit to get copy info only from the dirstate. This patch fixes that short-circuiting (and other places where context are being compared for equality). Differential Revision: https://phab.mercurial-scm.org/D7143