Skip invalid unicode in commit messages

When detecting color support, git-review attempts to decode to unicode
the git log output.  Whenever a commit message has invalid unicode, an
exception is thrown:

  File "git_review/cmd.py", line 366, in check_color_support
    (status, output) = run_command_status(test_command)
  File "git_review/cmd.py", line 130, in run_command_status
    out = out.decode('utf-8')
  File "python/2.7.6/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
  UnicodeDecodeError: 'utf8' codec can't decode byte 0xa0 in position
  197283: invalid start byte

Happens with https://github.com/composer/composer/commit/737fe32f4db
which contains \xA0.  Simply ask unicode decoder to replace the invalid
bytes with U+FFFD REPLACEMENT CHARACTER.

Change-Id: I041d1a9aab6fe00ad807407747c6263d577fba17
Merge "Skip invalid unicode in commit messages"