"git branch new A...B" and "git checkout -b new A...B" have been
taught that in their contexts, the notation A...B means "the merge
base between these two commits", just like "git checkout A...B"
detaches HEAD at that commit.
* dl/branch-from-3dot-merge-base:
branch: make create_branch accept a merge base rev
t2018: cleanup in current test
4ac8371a1c