Martin von Zweigbergk <martinvonz@google.com> [Fri, 10 Jan 2020 13:12:24 -0800] rev 44285
graft: let caller pass in overlayworkingctx to merge.graft()
Passing in a different `wctx` than `repo[None]` is useful because it
allows the caller to decide to not touch the working directory.
Differential Revision: https://phab.mercurial-scm.org/D8026