Deduplicate segments with the same tangent properly on mcf_graph::slope

[MiSawa]

This line doesn't seem to be correct me. Instead, we should update this variable by d.
Also, it would be better to name it cost_per_flow or something like that to avoid confusion.

ac-library/atcoder/mincostflow.hpp

Line 146 in a32a17b

prev_cost = cost;

A test case would be

TEST(MincostflowTest, SameCostPaths) {
    mcf_graph<int, int> g(3);
    ASSERT_EQ(0, g.add_edge(0, 1, 1, 1));
    ASSERT_EQ(1, g.add_edge(1, 2, 1, 0));
    ASSERT_EQ(2, g.add_edge(0, 2, 2, 1));
    auto expected = std::vector<std::pair<int, int>>{{0, 0}, {3, 3}};
    ASSERT_EQ(expected, g.slope(0, 2));
}

[yosupo06]

Thanks! Yes, this is a bug clearly.