overlayworkingctx: add `_checkexist(path)` This is in preparation to switch this class to inheriting (and being based off a) `commitctx` instead of a `workingctx`. `filectx` has no `exists` function, so this is how we'll fall back in that case. Differential Revision: https://phab.mercurial-scm.org/D1237

overlayworkingctx: fix a bad reference to `self._path` Differential Revision: https://phab.mercurial-scm.org/D1236

overlayworkingctx: track copy information in the context We can no longer do so in the dirstate, so add the functions to do so here. Differential Revision: https://phab.mercurial-scm.org/D1235