revlog-compression: fix computation of engine availability
We don't just need the engine to be define, we need it to be available and able
to do be used for revlog compression. Without this change, `zstd` could be
selected as a viable option for repository creation on platform where it is not
available.
Differential Revision: https://phab.mercurial-scm.org/D10325
@echo off
powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"