revlog-compression: update the config to be a list...
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
