Revision: Interpret a NULL rev_content_model as the default model

Revision: Interpret a NULL rev_content_model as the default model

If a revision using the default content model is saved, the
rev_content_model field will be set to NULL in order to save space.
However, Revision::getContentModel() was using Title::getContentModel()
when it found a NULL, which will return the current content model rather
than the default. So change that to use the default content model, as
returned by ContentHandler::getDefaultModelFor() rather than the current
one.

Change-Id: I7011812b6b131170b3c593917ad263bcba83898d