Comparing 2 merge request versions doesn't always show the expected diff

In the new Merge Request version feature, the comparison between version N and version M doesn’t always yield the expected result, because it’s a diff through the base (old...new) instead of a direct diff between old and new (old..new), which means it will show all changes between base and new when someone rebases or amends a commit, instead of showing the diff between old and new

/cc @dzaporozhets