Phil Cohen <phillco@fb.com> [Thu, 07 Dec 2017 16:07:06 -0800] rev 35323
overlayworkingctx: inherit from committablectx instead of workingctx
We should find a better name for this context at some point. (mutablememctx?)
Differential Revision: https://phab.mercurial-scm.org/D1240