Switch file uploader in "arc diff" to use ConduitEngine

Summary: Fixes T13498. Currently, arc diff may invoke the file upload flow using an older variation of the API. Both the modern and fallback clients build an Engine, so switching to the modern API is trivial.

Test Plan:

• Created a local commit with a 1MB binary file.
• Ran arc diff --only.
• Before patch: fatal on old API call.
• After patch: clean upload.

Maniphest Tasks: T13498

Differential Revision: https://secure.phabricator.com/D21043