Don't show continuation lines in "stacked action" comment previews

Summary:
Ref T10698. Currently, we show the spacer/continuation lines around previews, but these don't make sense in previews.

(Other stuff also uses this code so I can't simply remove spacer.)

Test Plan:
Before:

{F1199924}

After:

{F1199925}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10698

Differential Revision: https://secure.phabricator.com/D15562