Merge multiple Auditors transactions from Herald

Summary:
Fixes T12302. Currently, we aren't merging multiple "AddAuditors" transactions correctly.

This can occur when Herald triggers multiple auditor rules.

Instead, merge them.

Test Plan:

• Wrote two different Herald rules that add auditors.
• Pushed a commit which triggered them.
• After the change, saw all the auditors get added correctly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12302

Differential Revision: https://secure.phabricator.com/D17403