mergestate: extract a base class to be shared by future memmergestate
This extracts a new base class from `mergestate` and leaves all the
vfs-touching code in `mergestate`.
Differential Revision: https://phab.mercurial-scm.org/D9039
@echo off
powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"