Fix resolving expression with ~ and ^ than extends beyond history

Fix resolving expression with ~ and ^ than extends beyond history

resolve("foo~X") where X is greater than the distance from foo to the
root should return null, but 2a2362fb introduced a bug causing it to
either return resolve("foo") or NPE. Add a test for the correct
behavior.

Also add an analogous test for foo^X where X is greater than the
number of parents (which was not broken by that commit).

Change-Id: Ic580081ece57c8c2df29b652897b425ecb34e11f