R1:11f1268e993f

don't throw BadMethodCallException on diffs that add images

Summary:
I don't know if there is something more sinister going on
under the covers, but we have a couple of diffs that trigger:

Unhandled Exception ("BadMethodCallException")
Call to a member function getMetadata() on a non-object

when the diff page is handling its async render calls. One diff
in particular has multiple image adds and thus has a stack of of these
error dialogs to close.

This isn't a new regression, we just haven't gotten…