This component (and indeed all of the diffs app)
used to rely on `highlighted_diff_lines` whether you
were looking at the inline view or not.

When the split diffs was being written, most places
that did this were caught. This is one place that slipped
through the cracks since there were no tests checking
the behavior.

The update adds a generic "getPreviousLine" that uses
the diff view type to switch, and adds a bunch of tests
that will fail if the code is checking the wrong line type.