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
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.