Upgrade "arc upload" to use SHA256
Summary: Fixes T12464. Moves "arc upload" to SHA256 where applicable.
Test Plan: Ran arc upload against a server with D17620 twice, saw it skip the actual upload the second time.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12464
Differential Revision: https://secure.phabricator.com/D17622