context: clarify why we don't compare file contents when nodeid differs

See previous commit for timing information.

mercurial/context.py

@@ -140,15 +140,15 b' class basectx(object):'
                 added.append(fn)
             elif node2 is None:
                 removed.append(fn)
-            elif node2 != _newnode:
-                # The file was not a new file in mf2, so an entry
-                # from diff is really a difference.
+            elif self.rev() is not None:
+                # When comparing files between two commits, we save time by
+                # not comparing the file contents when the nodeids differ.
+                # Note that this means we incorrectly report a reverted change
+                # to a file as a modification.
                 modified.append(fn)
             elif flag1 != flag2:
                 modified.append(fn)
             elif self[fn].cmp(other[fn]):
-                # node2 was newnode, but the working file doesn't
-                # match the one in mf1.
                 modified.append(fn)
             else:
                 clean.append(fn)