Applies a minor performance improvement when both sides of the diff are
labeled by an unsupported language. Previously, the GR-SYNTAX-LAYER
`process` method would needlessly traverse the entire diff when the
languages were not in the support map. With this change, the `process`
method resolves early in such a scenario.
Change-Id: I8e5ef9fa410facdd60ca33c234ced682ffb182d7