For use cases that mix annexed files with files stored in git, the
annex.largefiles config is more important in v6 repositories than before,
since it configures the behavior of `git add` and even `git commit -a`. To
make it possible to set annex.largefiles so it'll stick across clones of
a repository, I have now made it be supported in `.gitattributes` files
as well as git config.

Setting it in .gitattributes looks a little bit different, since
the regular .gitattributes syntax can be used to match on the filename.

	* annex.largefiles=(largerthan=100kb)
	*.c annex.largefiles=nothing

It seems there's no way to make a git attribute value contain whitespace.
So, more complicated annex.largefiles expressions need to use parens to
break up the words.

	* annex.largefiles=(largerthan=100kb)and(not(include=*.c))
