Phil Cohen <phillco@fb.com> [Thu, 07 Dec 2017 13:20:47 -0800] rev 35304
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