Unfortunately you can't use Sapling with Mercurial repositories. There are too many differences at this point. This does not use hg-git under the hood. Sapling

Unfortunately you can't use Sapling with Mercurial repositories. There are too many differences at this point. This does not use hg-git under the hood. Sapling's internal structure differs from Mercurial in substantial ways, and we've built some cleaner layering that allowed us to shim Git in under our storage layer. This also means that we read and write directly to the git repo, instead of duplicating and importing all the data like hg-git did. This has some nice benefits, like the hashes you see in the output are actually Git hashes.