outdated - Show diff of packages for outdated / upgrade --noop to review changes

[kmindi]

As a user I would like to be able to find out what changed in packages that are about to be updated to find out if there were changes that might be malicious, e.g. download url is suspicious for the package or the install script performs other steps than previously.

Acceptance tests:

• The outdated command shows a hint to view the diff of the packages files that are outdated
• The upgrade --noop command shows a hint to view the diff of the packages files that are outdated
• There is a switch for the outdated and upgrade --noop command that creates a diff of the packages files that are outdated/about to be upgraded

[ferventcoder]

@kmindi this could be pretty interesting.