R1:6a6279705671

Fix some issues with Diffusion file data limits

Summary:
See <https://discourse.phabricator-community.org/t/files-created-from-repository-contents-slightly-over-one-chunk-in-size-are-truncated-to-exactly-one-chunk-in-size/988/1>. Three issues here:

- When we finish reading `git cat-file ...` or whatever, we can end up with more than one chunk worth of bytes left in the internal buffer if the read is fast. Use `while` instead of `if` to make sure we write the whole buffer.
- Limiting output with…