Update merge request overview tab changes count to use merge head

What does this MR do and why?

Updates the count on the changes tab to use the the merge head diff instead of the full diff. This can fix issues where the tab changes number after the user clicks on it.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After
Screen_Recording_2024-08-08_at_11.17.56	Screen_Recording_2024-08-08_at_11.20.14

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.