Define `diff` as a local variable

Summary: This should be a local variable, not a global variable. This silences a few JSHint warnings.

Test Plan: `arc lint`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11070