R1:7556a70280c8

Prevent Files from requiring infinite policy checks

Summary:
Fixes T6726. Currently, a file may be attached to itself (or to other files, ultimately forming a loop). In this case, we currently run around the loop forever trying to load all the files.

Instead, decline to load objects if we're inside a query which is already loading them. This produces the right policy result //and// completes in finite time.

Test Plan:
- Looped two files by writing `{F123}` and `{F124}` on the other files, respectively.