Fix incorrect diff file property reference

Before, the diffs *always* had both parallel and inline
diffs, so this should have never hit the else block.
Now that the split diffs feature is enabled, there are
cases (about 50% of the time) where the else is entered.

The test for inline diff lines was then trying to iterate over
the parallel lines, which of course don't exist in a lot of
situations.

app/assets/javascripts/diffs/store/utils.js

@@ -498,7 +498,7 @@ export const allDiscussionWrappersExpanded = diff => {
       }
     });
   } else if (diff.highlighted_diff_lines) {
-    diff.parallel_diff_lines.forEach(line => {
+    diff.highlighted_diff_lines.forEach(line => {
       if (line.discussions.length) {
         discussionsExpandedArray.push(line.discussionsExpanded);
       }