pkgctl diff: Provide namcap diff

For some packages, the namcap output is very noisy. E.g. for mesa, a heavily split package, the output is quite big and consists entirely of unfixable/hard-to-fix/false-positive errors/warnings. It's easy to miss namcap reporting something actually interesting.

I think it would be useful to provide a diff of the namcap output. I'm not sure how this could be implemented, as namcap is sensitive to the environment.