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.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}