Fix duplicated bottom match line

What does this MR do?

Fix the issue of merge request diffs (in parallel mode) contains multiple bottom match lines. Enable feature by default.

Cause and solution

The bug is caused when diff_lines_for_serializer gets called many times, because it modifies @highlighted_diff_lines.

Due to some tech debt, a proper fix would require more time. This is a quick fix by memoize the diff_lines_for_serializer so it only adds match line at most once.

What are the relevant issue numbers?

Closes #59147 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides