`helm diff revision` should default to comparing latest revision with previous revision

[gitfool]

Currently I need to specify at least one revision to compare with the latest revision:

helm diff revision [flags] RELEASE REVISION1

A common case is to compare the latest revision with the previous revision, so it would be nice to not have to lookup the revisions first to determine the previous revision number:

helm diff revision [flags] RELEASE

[avilaton]

using something like we have in git would be great

helm diff revision [flags] RELEASE HEAD^

then we could do whatever we want with that, like HEAD~3 or any weirdness.