By definition, the initial commit in a repository has no parents. You can
exploit that fact and use rev-list to find the initial commit; a commit
with no parents.
$ git rev-list --max-parents=0 HEAD
The rev-list command lists all commits in reverse chronological order. By
restricting them to those with at most 0 parents, you are only going to get
root commits. Generally, a repository will only have a single root commit,
but it is possible for there to be more than one.
See man git-rev-list for more details.