mdiff: mark diffopts as having dynamic attributes

mdiff: mark diffopts as having dynamic attributes

This class looks like we could move it to being an attrs and life
would be better, but let's do that later.

Differential Revision: https://phab.mercurial-scm.org/D7284

--- a/mercurial/mdiff.py	Wed Nov 06 15:29:08 2019 -0500
+++ b/mercurial/mdiff.py	Wed Nov 06 15:35:21 2019 -0500
@@ -38,6 +38,7 @@
 splitnewlines = bdiff.splitnewlines
 
 
+# TODO: this looks like it could be an attrs, which might help pytype
 class diffopts(object):
     '''context is the number of context lines
     text treats all files as text
@@ -52,6 +53,8 @@
     upgrade generates git diffs to avoid data loss
     '''
 
+    _HAS_DYNAMIC_ATTRIBUTES = True
+
     defaults = {
         b'context': 3,
         b'text': False,