Remember merges to save time.

Previously Zuul had to merge the entire change tree for each change
in the DependentPipeline. Remember previous merges to save time and
only merge changes against their project and branch.

Co-Authored-By: James E. Blair <jeblair@openstack.org>

Change-Id: I8105ac3aa4e4697e90e8d7fac1e54c5614f79daf
Reviewed-on: https://review.openstack.org/22504
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins