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 putting the entire raw diff in $raw_diff rather than using a rope/stream) but will likely survive diffs in the hundreds-of-megabytes range for now.

• Added str_repeat('x', 1024 * 1024 * 9) to the $raw_diff to create a 9MB+ diff.
• Configured file storage with no engine explicitly configured for >8MB chunks (i.e., "reasonably").
• Clicked "Download Raw Diff".
• Before: misleading file storage engine error ("no engine can store this file").
• After: large, raw diff response.