Allow diffing against initial (blank) package version

[nickstenning]

Currently the history page only allows diffing between different versions of a package, but there doesn't appear to be any easy way to see the changes introduced by the first version of a package.

I'm requesting the ability to diff against a "blank slate" initial state of a project, so I can see the content of the first project commit.

Not sure if this is a vdm feature, so I'm putting this ticket in against ckan.