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.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true