Skip first-parent check if upstream has diverged

	return proc.stdout.strip() or None

def is_ancestor(head_ref: str, other_ref: str) -> bool:

	"""

	Return True if "other_ref" is an ancestor of "head_ref"

	Note that "head_ref" refers to the head of a DAG graph to check, not a named reference

	in the refs/heads/ namespace.

	"""

	cmd = ["git", "merge-base", "--is-ancestor", other_ref, head_ref]

	return 0 == run(cmd).returncode

def check(ref: str, upstream: str) -> None:

	"""

	Exit with 1 if "upstream" can reach first-parent commits not reachable from "ref"

	Skipped if "upstream" is not an ancestor of "ref".  This would indicate rebasing which

	should be checked separately.

	"""

	if not is_ancestor(ref, upstream):

		return

	head_commits = get_fp_commits(ref)

	upstream_commits = get_fp_commits(upstream)

	if not upstream_commits.difference(head_commits):