fix: unicode filename causes IOError error

Fix following error when the filenames contains unicode characters.

    File "c:\Python27\lib\site-packages\dulwich-0.9.1-py2.7.egg\dulwich\index.py", line 433, in build_index_from_tree
    f = open(full_path, 'wb')
    IOError: [Errno 22] invalid mode ('wb') or filename: ...