Add loaded file name to Scuba.

Summary: This allows us to track how many people are loading which states +
how long it takes for them to do that compared to starting from a fresh state

Reviewed By: @jwatzman

Differential Revision: D1679348

Signature: t1:1679348:1415908364:a7f0c3a1982db5cb806f9e5c5221d1aa682493af