Fix reverting commit language

Summary: I think the language here is inverted? On a commit which is reverted, the text states $author added a reverted commit: $commit. In reality, $commit is the reverting commit, not the reverted commit.

Test Plan: Think about English.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13067