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.
$gitrev-list--max-parents=0HEAD
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.