Ref T2222. Currently, Differential has a fairly hairy piece of logic to parse object lists, like Reviewers: alincoln, htaft. Extract, generalize, and cover this.

• Some of the logic can be simplified with modern ObjectQuery stuff.
• Make @username the formal monogram for users.
• Make list@domain.com the formal monogram for mailing lists.
• Add test coverage.

• Ran unit tests.
• Called differential.parsecommitmessage with a bunch of real-world inputs and got sensible results.