Support formatting of subseconds

[patrickroocks]

The formatting symbol {%S} should also print the subseconds, e.g., fmt::format("{:%S}", std::chrono::microseconds(1234567)) should print "01.234567".

Didn't implement it for localized output as this localized output looks like black magic to me... may be we can omit it for the localized formatting for the first time?

[vitaut]

Please rebase.

[patrickroocks]

Rebased on master.

[vitaut]

Thanks for the PR.

[patrickroocks]

I reworked everything again.

• The work regarding subseconds was already done in the chrono_formatter for formatting durations. I pulled the code out of that and put in helper functions write_sub_sec and format_sec_fractional used in the tm_writer and the chrono_formatter.
• I had to move around many helper functions to make them available for write_sub_sec and format_sec_fractional and implicitly for the tm_writer.
• This implicitly supports arbitrary ratios for the Period.
• I found a bug for printing durations with floating point types. The unit test
  EXPECT_EQ(fmt::format("{:%S}", std::chrono::duration<double>{1.5}), "01.500000");
  didn't succeed, instead "01" was given as output. I changed the number of digits to at least 6, if the Rep type is fractional and round(value) != value.

[vitaut]

Mostly looks good, just a few more minor comments.

[vitaut]

Thank you!

[vitaut]

Fuzzer discovered an issue with this change. The following code

#include <fmt/chrono.h>

int main() {
  fmt::format("{:%S}", std::chrono::duration<char, std::ratio<1, 100> >(0x80));
}

now results in an assertion failure:

    frame #7: 0x000000010000dddb a.out`fmt::v9::detail::assert_fail(char const*, int, char const*) + 59
    frame #8: 0x000000010000ac3e a.out`long long fmt::v9::detail::to_nonnegative_int<long long, long long, 0>(long long, long long) + 94
    frame #9: 0x000000010000a8dd a.out`void fmt::v9::detail::write_fractional_seconds<char, std::__1::back_insert_iterator<fmt::v9::basic_memory_buffer<char, 500ul, std::__1::allocator<char>>>, std::__1::chrono::duration<char, std::__1::ratio<1l, 100l>>>(std::__1::back_insert_iterator<fmt::v9::basic_memory_buffer<char, 500ul, std::__1::allocator<char>>>&, std::__1::chrono::duration<char, std::__1::ratio<1l, 100l>>) + 173

[phprus]

@vitaut
Fix: #3143

[patrickroocks]

Thanks for the fix and sorry that I have overseen this!

[vitaut]

Here's a link to the oss-fuzz report for future reference: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=52380.