revlog: adapt the `reading` check for `unionrepo`
We cannot just rely on the length check for the `unionrepo` as the local revlog
might be empty while the other revlog contains data. In addition, we need to
also open the second revlog for reading when needed.
@echo off
powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"