Fix related changes after rebase

Given a stack of two commits C->P (child depends on parent), if P is
rebased or updated C should still considered to be a related change,
even if C has not yet been rebased onto P' (new P).

This was broken because we did not include all permutations of P
into the seed queue.

Bug: issue 2073
Change-Id: Ife954810ece39f44630b8c17a4f1b70192bd1611