Support appending arbitrary lines to DisplayRef output

Summary:
Ref T13546. Several substeps in the new "arc land" flow benefit from this. For example:

• When prompting "land revisions you don't own?", it's used to show authors.
• When prompting "land revisions in the wrong state?", it's used to show the current states.

Test Plan: Ran future "arc land" workflows, got relevant contextual information via this mechanism.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21308