How to reference the initial commit?

Assuming the first parent of a merge is the main line of development:

git rev-list --reverse --topo-order --first-parent HEAD | sed 1q

Not sure what you're trying to do here, but some git commands take --root as an option, in order to reference the root of the commit tree, e.g.

git rebase --interactive --root main

git rev-list HEAD | tail -n 1 is a more stable option.

Do not use git-log for scripting: use either git-rev-list, or git-log with specified custom format (--format=*<sth>* option).

There is additional problem with your question: there can exist more than one such TAIL root commit (parentless commit) in a repository (even if we discount disconnected branches, such as 'html', 'man' and 'todo' in git.git repository). This is usually result of joining separate projects in one, or using subtree merge of separately developed subproject.

For example git repository has 6 root commits: git-gui, gitk (subtree-merged), gitweb (merged in, no longer developed separately), git mail tools (merged very early in project history), and p4-fast-export (perhaps accidental). That is not counting roots of 'html and 'man' branches, "convenience" branches which contains pre-generated documentation, and 'todo' branch with TODO list and scripts.

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If you have git 1.7.4.2 or newer, you can use the --max-parents option:

$ git rev-list --max-parents=0 HEAD

Otherwise, you can get list of all parentless (root) commits accessible from current branch using:

$ git rev-list --parents HEAD | egrep "^[a-f0-9]{40}$"