Fix Merge Always and Merge If Necessary strategies

Second and additional changes in the same submit attempt must be
built on the previously constructed merge, not the branch tip.

Include tests for these strategies and the same bug in Cherry Pick
that was fixed by cd60499cd65812d926806e49d15a6852c7798871.

Change-Id: I62524b39ee4104ab53b36c75b64c02b20b4d5e94
6 files changed