diffutil: rewrite an ersatz ternary operator for building diffopts.text

Pytype tends to get confused by these, but the real problem here is that if the
caller passes `opts={}` as TortoiseHg has been doing, that is set as the value
for `diffopts.text` when in reality it expects a boolean.  When `None` is passed
explicitly, the default value in `mdiff.defaultopts` is assigned.

mercurial/diffutil.py

@@ -120,7 +120,7 b' def difffeatureopts('
         )
         buildopts[b'ignorewseol'] = get(b'ignore_space_at_eol', b'ignorewseol')
     if formatchanging:
-        buildopts[b'text'] = opts and opts.get(b'text')
+        buildopts[b'text'] = None if opts is None else opts.get(b'text')
         binary = None if opts is None else opts.get(b'binary')
         buildopts[b'nobinary'] = (
             not binary