The merge_db_schema project will resolve conflicts in the version of the
schema, but it does not resolve any remaining conflicts in a particular
way. This means that if there is a conflict further down the schema
file, Git may sometimes fail to resolve them.

This commit vendors some of the code of the mentioned project, and
changes it to use the "ours" strategy for resolving any further
conflicts. This in turn should prevent the Merge Train from failing when
there is a conflict in db/schema.rb.