两条分支各能追溯到几次提交

还是那张提交图。ancestors 求的是"从某次提交出发，顺着父提交一路能走到的全部提交（含自己）"。运行下面这段程序：

def ancestors(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

g = {'3f2a91c': [],
     '7b4e2d0': ['3f2a91c'],
     'a1c5f83': ['7b4e2d0'],
     'e90d417': ['7b4e2d0'],
     '5c8b206': ['e90d417']}
print(str(len(ancestors(g, 'a1c5f83'))) + "/" + str(len(ancestors(g, '5c8b206'))))