New annex.largefiles setting, which configures which files `git annex add` and the...

New annex.largefiles setting, which configures which files `git annex add` and the assistant add to the annex.

I would have sort of liked to put this in .gitattributes, but it seems
it does not support multi-word attribute values. Also, making this a single
config setting makes it easy to only parse the expression once.

A natural next step would be to make the assistant `git add` files that
are not annex.largefiles. OTOH, I don't think `git annex add` should
`git add` such files, because git-annex command line tools are
not in the business of wrapping git command line tools.