Don't put a spacer line on the bottom of previews
Summary: Fixes T8182. See screenshot in that task. We currently render a line to nowhere at the bottom of these previews. Instead, only render a line at the top.
Test Plan:
Also looked at a couple other applications that use this and they looked correct.
Reviewers: btrahan, lpriestley, chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T8182
Differential Revision: https://secure.phabricator.com/D12905