Stop indexing the chunk data objects for large Files stored in multiple chunks

Summary:
Ref T13164. See PHI766. Currently, when file data is stored in small chunks, we submit each chunk to the indexing engine.

However, chunks are never surfaced directly and can never be found via any search/query, so this work is pointless. Just skip it.

(It would be nice to do this a little more formally on IndexableInterface or similar as isThisAnIndexableObject(), but we'd have to add like a million empty "yes, index this always" methods to do that, and it seems unlikely that we'll end up with too many other objects like these.)

Test Plan:

• Ran bin/harbormaster rebuild-log --id ... --force before and after change, saw about 200 fewer queries after the change.
• Uploaded a uniquely named file and searched for it to make sure I didn't break that.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19563