NEWS: Document "nmbug: Translate to Python"

For more details, see the commit message for 7f2cb3be (nmbug:
Translate to Python, 2014-10-03).  I realized while writing this that
the 7f2cb3be commit message has:

  * 'nmbug log' now execs 'git log', as there's no need to keep the
    Python process around once we've launched Git there.

But we dropped that exec in favor of the subprocess approach between
v3 and v4, I just forgot to update the commit message [1].

[1]: id:e630b6763e9d0771718afee41ea15b29bb4a1de8.1409935538.git.wking@tremily.us
     http://article.gmane.org/gmane.mail.notmuch.general/19007
NEWS: mention the change in default build flags

It blows things up by a factor of six or so, so it's worth giving
people a heads up. It won't effect e.g. Debian, that already builds
with -g and then strips.