revlog-compression: use zstd by default (if available)...
revlog-compression: use zstd by default (if available)
As see in changeset bb271ec2fbfb, zstd is 20% to 50% faster for reading and
writing.
Use take advantage of the new config behavior to try zstd by default, falling
back to zlib is zstd is not available on that plateform.
Differential Revision:
https://phab.mercurial-scm.org/D10326
