Main idea
Given a tree like this one: 
A - * - * - * - * origin/master
  \ * - B - * - * origin/feature
         \- C - D topic

when we rbt post, it would like to have three things, 

            'base':        A revision to use as the base of the resulting diff. 

            'tip':         A revision to use as the tip of the resulting diff.

            'parent_base': (optional) The revision to use as the base of a       

                           parent diff.                                          

Now, parent_base has to be in the remote repository, in other words, a ancestor of a commit of any branch origin/*.  We would also like parent_base to be as young an ancestor of tip as possible, given the first constraint.
I've worked out a git-rev-list query to find this.
git rev-list $TIP --not --remotes=origin | tail -1

should give the oldest ancestor that is not reachable from a public branch, so it's parent will be the best parent base.

Now with tests!

test_diff_finding_parent is a new test, and I had to modify test_parse_revision_spec_no_args_parent to pass, since the parent base is no longer the same after my modifications.