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 diff), so we call the method, it shoves the data in Files, and then we go pull it out of Files.

To support this, take the "shove the output of a Future into Files" logic and put it in a new base FileFuture query. This will let me make RawDiffQuery share the logic more easily.

Test Plan: Browsed Diffusion, ran diffusion.filecontentquery to fetch file content.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10423, T11524

Differential Revision: https://secure.phabricator.com/D16458