Allow "arc upload" to work correctly if it can not hash content
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.
Reviewed By: chad
Maniphest Tasks: T12464
Differential Revision: https://secure.phabricator.com/D17621