py3: replace file() with open() in test-convert-mtn.t
While I was here, I added b'' prefixes to make sure we write things correctly in
bytes mode on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D2111
@echo off
powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"