R1:771579496fc1

Make logic for streaming VCS stuff directly to Files more reusable

Summary:
Ref T11524. Ref T10423. Earlier, I converted `diffusion.filecontentquery` to put the actual file content in Files, then return a PHID for the file, instead of trying to send the content over Conduit.

In T11524, we have a similar set of problems with diffs that contain non-UTF8 data (and, in T10423, diffs that are simply enormous).

I want to provide an API method to do the same sort of thing with diff output (like from `git…