rhg: tweak rhg fallback code structure
The goal here is to share the code that accesses the config setting,
but also move it to the top-level, so that we can more easily change
this code later.
(in fact in Jane Street we want to make it choose chg in some cases)
@echo off
powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"