R1:14b076578f8e

In "Download Raw Diff", engage the chunk engine to handle 8MB+ changes

Summary:
Fixes T13313. The "Download Raw Diff" workflow in Differential currently uses an older way of interacting with Files that doesn't engage the chunk engine and can't handle 8MB+ files.

Update to `IteratorFileUploadSource` -- we're still passing in a single giant blob, but this approach can be chunked.

This will still break somewhere north of 8MB (it will break at 2GB with the PHP string limit if nowhere sooner, since we're…