log.py, minimal/do: don't use ansi colour codes if $TERM is blank or 'dumb'

Apparently emacs sets TERM=dumb in its tty simulator, so even though
isatty() returns true, we shouldn't use colour codes.  (emacs is therefore
lame. But we knew that.)

log.py

@@ -8,7 +8,7 @@ YELLOW = ""
 BOLD   = ""
 PLAIN  = ""
 
-if sys.stderr.isatty():
+if sys.stderr.isatty() and (os.environ.get('TERM') or 'dumb') != 'dumb':
     # ...use ANSI formatting codes.
     RED    = "\x1b[31m"
     GREEN  = "\x1b[32m"