I don’t need to isolate all the implementations, anymore. I can see the history of a given implementation by constraining git log to its folder. I used a single git merge command that created a single merge node in the Git repo with multiple ancestors.

git merge \
    grails_mysql \
    grails_mongo \
    grails_neo4j \
    node_mongo \
    node_graphql_federation \
    micronaut_neo4j \
    rails_mysql_graphql_user_errors