Root commits are commits with zero parents. If a commmit has no
parents it is the first commit in the repository. In general the root
commits should be unique for any given project, as the first commit
will be created at a different time, by a different user with its own
message. These root commits can be used as a "fingerprint" to
identify disjoint histories.
Change-Id: Id891dbc1f17c816cea404569578bb7635ff85cdb