Function get_rebase_fork_point

builtin/pull.c:646–684

* Given the repo and refspecs, sets fork_point to the point at which the * current branch forked from its remote-tracking branch. Returns 0 on success, * -1 on failure. */

 * -1 on failure.
 */
static int get_rebase_fork_point(struct object_id *fork_point, const char *repo,
		const char *refspec)
{
	int ret;
	struct branch *curr_branch;
	const char *remote_branch;
	struct child_process cp = CHILD_PROCESS_INIT;
	struct strbuf sb = STRBUF_INIT;

	curr_branch = branch_get("HEAD");
	if (!curr_branch)
		return -1;

	if (refspec)
		remote_branch = get_tracking_branch(repo, refspec);
	else
		remote_branch = get_upstream_branch(repo);

	if (!remote_branch)
		return -1;

	strvec_pushl(&cp.args, "merge-base", "--fork-point",
		     remote_branch, curr_branch->name, NULL);
	cp.no_stdin = 1;
	cp.no_stderr = 1;
	cp.git_cmd = 1;

	ret = capture_command(&cp, &sb, GIT_MAX_HEXSZ);
	if (ret)
		goto cleanup;

	ret = get_oid_hex(sb.buf, fork_point);
	if (ret)
		goto cleanup;

cleanup:
	strbuf_release(&sb);
	return ret ? -1 : 0;
}

/**
 * Sets merge_base to the octopus merge base of curr_head, merge_head and