Fix displaying gap context when showing first lines

Summary: Show First 20 Lines doesn't display gap context and emits error.

Test Plan: Showed first 20 lines, last 20 lines, middle 20 lines - saw context and no error.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3616