Add test for pushing new patch set directly

In version 2.11-rc0 if a change is pushed for review, and then a new
patch set is pushed directly, the change is not set to merged and
remains open.

This was caused by a NullPointerException, which was already fixed
in [1] after being found through a different use case.  That fix is
already included since 2.11-rc1.

Add an acceptance test for pushing a new patch set directly, to catch
any similar problems in future.

[1] https://gerrit-review.googlesource.com/#/c/65200

Bug: Issue 3237
Change-Id: I045af58a94709276caba9314d77eedd9d84a4a3b
1 file changed