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.

--- a/tests/test-repo-compengines.t	Tue Apr 16 15:10:16 2019 +0200
+++ b/tests/test-repo-compengines.t	Wed Mar 27 18:27:03 2019 +0100
@@ -44,9 +44,9 @@
   $ cd zstd
   $ cat .hg/requires
   dotencode
-  exp-compression-zstd
   fncache
   generaldelta
+  revlog-compression-zstd
   revlogv1
   sparserevlog
   store