R1:cb99396c64dc

Make the "Is this JSON?" DocumentEngine heuristic a little tighter

Summary:
See PHI749. Ref T13164. We currently misdetect files starting with `[submodule ...` as JSON.

Make this a bit stricter:

- If the file is short, just see if it's actually literally real JSON.
- If the file is long, give up.

This should get the right result in pretty much all the cases people care about, I think. We could make the long-file guesser better some day.

Test Plan: Detected a `[submodule ...` file (no longer JSON)…