Preparation for a new merge strategy.
* en/merge-ort-api-null-impl:
merge,rebase,revert: select ort or recursive by config or environment
fast-rebase: demonstrate merge-ort's API via new test-tool command
merge-ort-wrappers: new convience wrappers to mimic the old merge API
merge-ort: barebones API of new merge strategy with empty implementation