Fix coverage line index lookup in diffusion browser

Summary: I believe this got clobbered in rP8b6edaa4e238a809fe78e6d14ad0705545f8179f. This index doesn't seem to be present in the line dictionary and we're now relying on $line_index for the current position.

Test Plan:
before {F1085522}
after {F1085521}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D15156