diff dict API bug fix

Summary:
I didn't realize createDiffDict was a public method when I
modified it, and I broke the API call in getrevision. This moves the
modification inside the method and reverts the method header back to
it's original form.

Test Plan: none

Reviewers: epriestley

Reviewed By: epriestley

CC: edward, aran, epriestley

Differential Revision: 883