graft: let caller pass in overlayworkingctx to merge.graft()...
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
