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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).