Fix search result highlight when scrolling

Summary:
The highlighted line of a search result is not updated correctly
when scrolling a screen with repeated lines.

Test Plan:
cat a file containing the same line repeated many times.
Search for a string, then scroll: the highlight line should
be displayed correctly.

{F6414451}

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D16820

(cherry picked from commit 3fc9fab7)