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) and a {"duck": "quack"} file (still JSON).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19544