Fixed bug resulting in duplicate field names in commit messages

Summary:
My earlier diff refactored some code without completely
respecting the semantics, sometimes resulting in duplicate field names
returned from differential.getcommitmessage. This fixes that.

Test Plan:
ran "arc diff" with diff causing the bug (commit message
had an empty Revert Plan: field) and verified no duplicate fields

Reviewed By: epriestley
Reviewers: epriestley
CC: aran, dpepper, epriestley
Differential Revision: 610