Fix duplicated bottom match line

Mark Chao requested to merge 59147-duplicate-match-line into master

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?

Edited by Mark Chao

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