compression: introduce an official `zstd-revlog` requirement...
compression: introduce an official `zstd-revlog` requirement
This requirement supersedes `exp-compression-zstd`. However, we keep support for
the old requirement.
Strictly speaking, we do not need to add a new requirement, there are no logic
change making "new" repo incompatible with mercurial client using a version
that support `exp-compression-zstd`.
The choice to introduce a new requirement is motivated by the following:
* The previous requirement was explicitly "experimental". Using it by default
could confuse users.
* adding support for a hypothetical third compression engine will requires new
code, and will comes with its own requirement tool.
* We won't use it as the default for a while since I do not think we support
zstd on all platform. I can imagine we'll gain another (unrelated but on my
default) requirement by the time we turn this zstd by default.
