合并完成后主线上有几次提交

合并顺利完成后，主线的历史里能追溯到几次提交？（承上一条路线那张图：两边各自往前走过，这次是三方合并。）运行下面这段程序：

g = {'3f2a91c': [], '7b4e2d0': ['3f2a91c'],
     'a1c5f83': ['7b4e2d0'], 'e90d417': ['7b4e2d0'],
     '5c8b206': ['e90d417'],
     '9d31e5f': ['a1c5f83', '5c8b206']}
def anc(g, c):
    seen = set()
    st = [c]
    while st:
        x = st.pop()
        if x in seen:
            continue
        seen.add(x)
        st.extend(g[x])
    return seen
print(len(anc(g, '9d31e5f')))