Allow "arc upload" to work correctly if it can not hash content

Summary:
Ref T12464. This is similar to D17619 and prepares us to move to SHA256 in the client.

Note that it's fine if arc and Phabricator disagree about hashing algorithms. We don't really trust the client anyway, so if things are mismatched clients will just end up transferring a bit more data instead of getting to cheat when Phabricator already has copies of data.

Test Plan: Ran arc upload, got a clean upload.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12464

Differential Revision: https://secure.phabricator.com/D17621