Fix LFS size computation with identical sizes

What does this MR do and why?

This MR fixes #339877 (closed)

Rails issue with the same kind of wrong sum calculation https://github.com/rails/rails/issues/33082

first commit adds a 3rd LFS object with the same size as 2nd object. It shows that tests fail guillaume.chauvel/gitlab@0895696f
2nd commit makes tests pass

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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Closes #339877 (closed)