dagop: add smartset interface to filectxancestors() The original filectx API is kept public since we'll need it to walk ancestor (rev, match) pairs efficiently. The current implementation scans ancestors twice for 'hg log -fp FILE'.

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