kpt pkg rebase

[bgrant0607]

Kpt makes it pretty easy to fork a fork of a fork, though it doesn't explicitly have the concept of a fork (see #2544).

I'd like to keep versioning simple, rather than trying to use something like semantic versioning: A.B.C.D.E.F...

For continuously delivered configuration, I am leaning towards a single version integer.

However, I may want to switch to another branch of the package inheritance at some point -- think of it as like a major version change.

To do that, I think we would need a "kpt pkg rebase" command to switch to a new upstream.

Without recording the branch points of the tree, I'm not sure how to implement the 3-way merge, but it's worth thinking about.

cc @justinsb