Treat git output as UTF-8

My problem is in <https://travis-ci.org/chewing/libchewing/jobs/9887331>, where the author contains UTF-8 character. Because the output of `locale.getpreferredencoding(False)` is `ANSI_X3.4-1968`, the `decode` will raise `UnicodeDecodeError`.

In this patch, I just assume the output of git is UTF-8 encoding, and using unicode interface to handle them. Although git can handle encoding other than UTF-8, UTF-8 is preferred encoding, and using legacy encoding will cause warning according to [git document](https://www.kernel.org/pub/software/scm/git/docs/git-commit.html#_discussion).