nmbug-status: Decode Popen output using the user's locale

nmbug-status: Decode Popen output using the user's locale

Avoid:

  $ ./nmbug-status --list-views
  Traceback (most recent call last):
    File "./nmbug-status", line 47, in <module>
      'cat-file', 'blob', sha1+':status-config.json'],
  TypeError: can't concat bytes to str

by explicitly converting the byte-stream read from Popen into a
Unicode string.  On Python 2, this conversion is str -> unicode; on
Python 3 it is bytes -> str.

_ENCODING is derived from the user's locale (or system default) in an
attempt to match Git's output encoding.  It may be more robust to skip
the encoding/decoding by using a Python wrapper like pygit2 [1] for
Git access.  That's a fairly heavy dependency though, and using the
locale will probably work.

[1]: http://www.pygit2.org/