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
after
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D15156