Fix cloning to path with unicode

[ulturt]

Fix cloning to directories with a path containing Unicode characters
Closes issue #920

[Byron]

Thanks a lot for improving yet another instance of GitPython's confused way of handling encodings.

For a moment I thought maybe it's generally helping if force_bytes would default its encoding keyword parameter to utf-8 instead of ascii, since ascii is a valid subset of utf-8, i.e. all ascii files are also utf-8 encoded files.

Then I was uncertain if it would actually end up working that way. Your advice is appreciated, maybe it's worth another PR if you think it should work.