Use default external encoding for tree entry names

Tree entry names are created with the ASCII encoding. I think git doesn't specify anything about file name encoding, so it may be good enough. But when your repository has entries in another encoding, you have to `force_encoding` on every entry name.

I patched rugged to use the default external encoding instead of "no encoding".
I'm not sure that's the best solution. Maybe an attribute on the Rugged module is better, or on the Repository object (but I don't know if we can access it from `rb_git_treeentry_fromC`).