Fixed bug service finding wrong diff line index

Fixes a bug where if the change is after the first line in a file
the outdated diff lines service would find the incorrect starting
index and then return an incorrect diff.

app/services/merge_requests/outdated_discussion_diff_lines_service.rb

@@ -15,7 +15,13 @@ module MergeRequests
 
     def execute
       end_position = position.line_range["end"]
-      diff_line_index = diff_lines.find_index { |l| l.new_line == end_position["new_line"] || l.old_line == end_position["old_line"] }
+      diff_line_index = diff_lines.find_index do |l|
+        if end_position["new_line"]
+          l.new_line == end_position["new_line"]
+        elsif end_position["old_line"]
+          l.old_line == end_position["old_line"]
+        end
+      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