patch: use field names instead of field numbers on scmutil.status

As part of my pytype adventures I want to make scmutil.status no longer a
subclass of tuple. This is part of that process.

Differential Revision: https://phab.mercurial-scm.org/D7401

mercurial/patch.py

@@ -2605,7 +2605,14 b' def diffhunks('
 
     if not changes:
         changes = ctx1.status(ctx2, match=match)
-    modified, added, removed = changes[:3]
+    if isinstance(changes, list):
+        modified, added, removed = changes[:3]
+    else:
+        modified, added, removed = (
+            changes.modified,
+            changes.added,
+            changes.removed,
+        )
 
     if not modified and not added and not removed:
         return []