internal/multilinediff: change string concatenation to use strings.Repeat instead

[artur-beznosyuk-verkada]

Fix for issue #283

This should guarantee exactly the same behavior but avoids edge case performance issues. I outlined few other suggestions in the task

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[artur-beznosyuk-verkada]

I signed CLA and triggered rescan

[braydonk]

Thanks for the PR. What effect did this have on the performance on that large file?

[braydonk]

Forgot to hit approve

[artur-beznosyuk-verkada]

Thanks for the quick acceptance :) with using Repeat the runtime on the same file went from ~60-70 minutes to about 10 seconds