Add a DiffusionRefDatasource for typeahead'ing branches, tags, bookmarks and refs

Summary: Ref T9952. This will let me put a "Branch: [____]" control on the "Land Revision" dialog so users can choose a branch to target.

Test Plan: Used /typeahead/class/ to vet basic behavior.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9952

Differential Revision: https://secure.phabricator.com/D14732