revlog-compression: update the config to be a list format.revlog-compression is now a list of engine, the first supported one is to be used. Doing this have several benefits: 1) this is fully backward compatible, config using a single entry will be read as a single item list, not changing any behavior. 2) This open the way to use zstd by default without impacting platform were it is not available. This will be done in a later changesets. Using zstd provide a significant performance boost explained in : bb271ec2fbfb. However zstd is not available in some cases, A notable example is the `--pure` version of Mercurial which doesn't come with zstd support. Differential Revision: https://phab.mercurial-scm.org/D8148

--- a/mercurial/configitems.py	Wed Feb 19 13:39:00 2020 +0530
+++ b/mercurial/configitems.py	Wed Feb 19 17:30:04 2020 +0100
@@ -759,7 +759,7 @@
 coreconfigitem(
     b'format',
     b'revlog-compression',
-    default=b'zlib',
+    default=lambda: [b'zlib'],
     alias=[(b'experimental', b'format.compression')],
 )
 coreconfigitem(