chrono asserts and errors for negative values

[pauldreik]

While investigating a fuzzer crash, I found that negative values do not seem to be handled correctly. Here are some test cases with comments, which I would appreciate could get added to the unit tests and fixed.
My problem is that the fuzzer trips on these, which stops me from working with the real problem I try to solve :-)

TEST(ChronoTest, Negative_case1) {
  // this prints --12345 instead of -12345
  EXPECT_EQ("-12345", fmt::format("{:%Q}", std::chrono::seconds(-12345)));
}
TEST(ChronoTest, Negative_case2) {
  // this works as intended
  EXPECT_EQ("-03:25:45",
            fmt::format("{:%H:%M:%S}", std::chrono::seconds(-12345)));
  // this prints "-25:25:25" instead of "-25:-25:-25"
  EXPECT_EQ("-25:-25:-25",
            fmt::format("{:%M:%M:%M}", std::chrono::seconds(-12345)));
}
TEST(ChronoTest, Negative_case3) {
  // this prints -s instead of s
  EXPECT_EQ("s", fmt::format("{:%q}", std::chrono::seconds(-12345)));
}
TEST(ChronoTest, Negative_case4) {
  // this works as intended.
  EXPECT_EQ("0.127", fmt::format("{:%S}",
                                std::chrono::duration<char, std::milli>{127}));
  // this triggers an assert in make_unsigned
  EXPECT_EQ("-.127", fmt::format("{:%S}",
                                std::chrono::duration<signed char, std::milli>{-127}));
}
TEST(ChronoTest, Negative_case5) {
  // this works as intended.
  EXPECT_EQ("-03:25", fmt::format("{:%R}", std::chrono::seconds(-12345)));
}
TEST(ChronoTest, Negative_case6) {
  // this works as intended.
  EXPECT_EQ("-03:25:45", fmt::format("{:%T}", std::chrono::seconds(-12345)));
}
TEST(ChronoTest, ShouldntCrash1) {
  try {
    // this triggers an assert in make_unsigned
    fmt::format("{:%R}", std::chrono::duration<char, std::mega>{2});
  } catch (...) {
  }
}
TEST(ChronoTest, ShouldntCrash2) {
  try {
    // this triggers an assert in make_unsigned
    fmt::format("{:%T}", std::chrono::duration<char, std::mega>{2});
  } catch (...) {
  }
}

[vitaut]

Negative cases 1, 3 and assertions should be fixed in 30bce6c.

I think case 2 is correct and fmt::format("{:%M:%M:%M}", std::chrono::seconds(-12345)) should produce "-25:25:25" for consistency with other cases (although the whole format is kinda meaningless).

I couldn't repro case 2. It was printing "-00.127" (not "-.127") as expected.

Thanks for catching all of these!