Fix a Harbormaster build log issue where too few header lines were returned when expanding logs in the middle

Summary: Ref T13088. This variable bled through from an earlier loop and caused us to drop some of the lines in the middle.

Test Plan: Clicked "Show More", got an equal number of header and footer lines.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19148