Fix an issue where text intradiff bodies may not render

Summary:
Ref T13523. In the caching layer, there's a tricky clause about filetypes that skips some body rendering behavior.

Provide file type information which at least has a better chance of representing all changes (e.g., an image file may be replaced with a text file, but this can not be represented by a single file type).

Formalize "hasSourceTextBody()", to mean the changeset parser should engage the change as source text.

Test Plan: Intradiffed text changes, saw the body render properly.

Maniphest Tasks: T13523

Differential Revision: https://secure.phabricator.com/D21210