rhg: make rhg recognise it supports zstd compression for revlogs
It already did, but was not aware of it.
Differential Revision: https://phab.mercurial-scm.org/D10324
#require black test-repo
Black needs the real USERPROFILE in order to run on Windows
#if msys
$ USERPROFILE="$REALUSERPROFILE"
$ export USERPROFILE
#endif
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`