From http://stackoverflow.com/questions/1549146/find-common-ancestor-of-two-branches
http://stackoverflow.com/questions/2816715/branch-from-a-previous-commit-using-git
$ git merge-base branch2 branch3
050dc022f3a65bdc78d97e2b1ac9b595a924c3f2
# create the branch via hash,
git branch branchname <sha1-of-commit>
#or by using a symbolic ref.
git branch branchname HEAD~3