UnicodeDecodeError when "View History..." is triggered and gitk isn't installed and zh_TW locale is used

[Vdragon]

Reproduce step

1. env LANGUAGE=zh_TW git cola
2. open a git repository
3. right-click a file in status widget and choose "View History..."

Traceback

Traceback (most recent call last):
  File "cola/widgets/status.py", line 952, in view_history
    cmds.do(cmds.VisualizePaths, selection.union(selection.selection_model()))
  File "cola/cmds.py", line 1803, in do
    return do_cmd(cls(*args, **opts))
  File "cola/cmds.py", line 1812, in do_cmd
    msg, details = utils.format_exception(e)
  File "cola/utils.py", line 38, in format_exception
    details = '\n'.join(details)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe5 in position 22: ordinal not in range(128)

Environment

OS

Ubuntu 14.04

Locale

LANG=zh_TW.UTF-8
LANGUAGE=zh_TW:zh_HK:zh_CN:en_US:en