Today I have learned two different usages of git command. First, to list the least common ancestor of two refs, we may use following command:

$ git merge-base --octopus <commit_1> <commit_2> ... <commit_n>

Please notice that --octopus is mandatory; otherwise, git will create a hypothetical commit M, which merges commit_2 .. commit_n, and then compute the common ancestor of commit_1 and M.

The git merge-base command will print the SHA1 of the least common ancestor. If such commit is not available, then git will return with error.

The second usage I have learned is that we can create a orphan branch with following commands:

$ git checkout --orphan <new-branch-name>
$ git reset
$ rm -rf *
... add files ...
$ git commit

The orphan branch will be completely independent of the existing branches. This is helpful when creating GitHub Pages branch.