mdiff: add a __str__ method to diffopts

This makes it easier to debug by just formatting the object into `%s` to see the
members and state, instead of the class and memory address.

mercurial/mdiff.py

@@ -94,6 +94,13 b' class diffopts:'
         opts.update(kwargs)
         return diffopts(**opts)
 
+    def __bytes__(self):
+        return b", ".join(
+            b"%s: %r" % (k, getattr(self, k)) for k in self.defaults
+        )
+
+    __str__ = encoding.strmethod(__bytes__)
+
 
 defaultopts = diffopts()