Fix "arc diff --raw-command --create"

Summary: Fixes T5082. We try to access the repository API in some cases when we don't have one.

Test Plan:

• Made revisions with "arc diff --raw-command --create".
• Made this revision, without "--raw-command".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5082

Differential Revision: https://secure.phabricator.com/D9165