Allow diffusion.filecontentquery to load data for arbitrarily large files

Summary:
Fixes T10186. After D14970, diffusion.filecontentquery puts the content in a file and returns the file PHID.

However, it does this in a way that doesn't go through the chunking engine, so it will fail for files larger than the chunk threshold (generally, 8MB).

Instead, stream the file from the underlying command directly into chunked storage.

Test Plan:

• Made a commit including a really big file: https://github.com/epriestley/poems/commit/4dcd4c492b31d82f0fec6f447cf3719b9cc079e0
• Used diffusion.filecontentquery to load file content.
• Parsed/imported commit locally.
• Used diffusion.filecontentquery to load content for smaller files (README, etc).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10186

Differential Revision: https://secure.phabricator.com/D15072