Don't include pkg/arvo/tests and pkg/urbit/tests when running
git-archive(1).
Additionally, move the export-ignore declaration for
pkg/urbit/hashtable_tests into the pkg/urbit/.gitattributes files. For
default operation, git-archive(1) requires that these declarations be
located on the tree being archived.
The 'index.js', 'tile.js', and 'index.css' files included in several
Arvo apps are compiled, minified JS or CSS. While they are technically
text, they are not usefully diffable.
This commit designates such files as 'binary' in .gitattributes, which
prevents git from attempting to create diffs for them in 'git log' and
'git show'.
