`gln` fails with non-ascii commit messages

[danvk]

$ gln
Traceback (most recent call last):
  File "/Users/danvk/github/git-helpers/log/git-lg", line 46, in <module>
    pieces.parse_log(['-n', number, branch])
  File "/Users/danvk/github/git-helpers/util/piece.py", line 142, in parse_log
    lines = out.decode().splitlines()
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 314: ordinal not in range(128)

The offending commit is this one:

commit c481c62bc996cfe22417621dfd96c4267eb1e740
Author: Marc Cornellà <marc.cornella@live.com>
Date:   Mon Aug 31 19:17:16 2015 +0200

    Refactor and clear up README file

Note the accent in the author.