patch.diffopts: allow a setting to be forced in plain mode

Upcoming patches will add an option that will almost certainly break diff
output parsers when enabled. Add support for forcing an option to something in
plain mode, as a fallback. Options passed in via the CLI are not affected,
though -- it is assumed that any script passing the option in explicitly knows
what it is doing.

mercurial/patch.py

@@ -1564,6 +1564,8 def diffopts(ui, opts=None, untrusted=Fa
             v = opts.get(key)
             if v:
                 return v
+        if forceplain is not None and ui.plain():
+            return forceplain
         return getter(section, name or key, None, untrusted=untrusted)
 
     return mdiff.diffopts(