merge-base 是哪一次提交

算出 main 和 feature 最近的共同祖先。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

def merge_base(g, a, b):
    both = ancestors(g, a) & ancestors(g, b)
    for x in both:
        others = both - {x}
        if not any(x in ancestors(g, y) for y in others):
            return x
    return None

g = {'3f2a91c': [],
     '7b4e2d0': ['3f2a91c'],
     'a1c5f83': ['7b4e2d0'],
     'e90d417': ['7b4e2d0'],
     '5c8b206': ['e90d417']}
print(merge_base(g, 'a1c5f83', '5c8b206'))