Fixes outdated diff lines returning incorrect results

The outdated diff lines service will sometimes return
incorrect results if the commented line number no longer exists in the
diff.
So instead we find the old diff line and then find the _next_ new
or unchanged line and use that as the index to display
the correct diff.

app/services/merge_requests/outdated_discussion_diff_lines_service.rb

@@ -15,12 +15,22 @@ module MergeRequests
 
     def execute
       line_position = position.line_range["end"] || position.line_range["start"]
-      diff_line_index = diff_lines.find_index do |l|
-        if line_position["new_line"]
-          l.new_line == line_position["new_line"]
-        elsif line_position["old_line"]
-          l.old_line == line_position["old_line"]
+      found_line = false
+      diff_line_index = -1
+      diff_lines.each_with_index do |l, i|
+        if found_line
+          if !l.type
+            break
+          elsif l.type == 'new'
+            diff_line_index = i
+            break
+          end
+        else
+          # Find the old line
+          found_line = l.old_line == line_position["new_line"]
         end
+
+        diff_line_index = i
       end
       initial_line_index = [diff_line_index - OVERFLOW_LINES_COUNT, 0].max
       last_line_index = [diff_line_index + OVERFLOW_LINES_COUNT, diff_lines.length].min