Fix an issue where any diff which could possibly be rendered as Jupyter decided to render as Jupyter
See PHI1468. Engine selection for diffs is currently too aggressive in trying to find a shared engine and will fall back a shared engine with a very low score, causing all ".json" files to render as Jupyter files.
Only pick an engine as a difference engine by default if it's the highest-scoring engine for the new file.
Test Plan: Viewed ".json" files and ".ipynb" files in a revision. Before, both rendered as Jupyter. Now, the former rendered as JSON and the latter rendered as Jupyter.
Differential Revision: https://secure.phabricator.com/D20850