Fix diff file tree incorrect counters

What does this MR do and why?

Fixes Diffs file tree has incorrect changes counters (#392634 - closed).

This MR removes v-once from the changes counter because the component can be re-used by the virtual scroller by passing a different prop and it expects that the component re-renders correctly. v-once causes a single render, so once the counter is rendered it will always stay the same, that's why we see wrong counters with virtual scrolling.

How to set up and validate locally

1. Open any MR with many files changed
2. Scroll the file tree
3. You should see correct change counters alongside file path

MR acceptance checklist

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