lfs: migrate most file filtering from threshold to custom filter
Migrate `lfs.threshold` to more powerful `lfs.filter` added by D4990618 so
people can specify what files to be stored in LFS with more flexibility.
This patch was authored by Jun Wu for the fb-experimental repo, to avoid using
matcher for efficiency[1]. All I've changed here is to register the new
'lfs.track' default so that the tests run cleanly, and adapt the subsequent
language changes. Migrating the remaining uses of 'lfs.threshold' can be done
separately since there's a fallback in place.
[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-December/109388.html
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