git-xbase: display a traceback when file writing fails

Otherwise we fail silently, which is hard to debug.

Signed-off-by: David Aguilar <davvid@gmail.com>
git-xbase: handle unicode in commit messages

Call a helper method instead of coercing to str().  We'd need to do this
eventually anyways if we were to move to Python 3.

Signed-off-by: David Aguilar <davvid@gmail.com>