Fix support for dyff as external diff program

[timebertt]

dyff has some special integration in place that makes it compatible with kubectl diff, see homeport/dyff#149.
This is because, with KUBECTL_EXTERNAL_DIFF="dyff between", kubectl calls dyff <from> <to> between, but dyff expects dyff between <from> <to>.
kubectl diff uses different directories for from and to, which is automatically handled by dyff: https://github.com/homeport/dyff/blame/c382d5132c86d2280335f4cb71754ab20776a85a/internal/cmd/root.go#L85-L98

While kubectl revisions diff uses the same implementation for calling an external diff program like kubectl diff, the difference is that kubectl revisions diff doesn't use dedicated directories for from and to.

This PR changes the plugin to use dedicated directories as well, so that dyff works as an external diff program.
This might seem a bit hacky. Instead, one could also write a custom implementation that doesn't append but prepend args like between. However, I'm reluctant to stop reusing kubectl's implementation because it might reveal further discrepancies in the future.

The PR also adds a dedicated e2e test that ensures kubectl revisions diff works with the recommended setup for using dyff with kubectl, i.e., that it works with the same KUBECTL_EXTERNAL_DIFF setting.