`group_digits`does not work with negative integers

[lriesterer]

group_digits does not work with negative integers.

I would expect fmt::group_digits(-12345) to produce -12,345 and not 18,446,744,073,709,539,271

Here is the code to reproduce: https://godbolt.org/z/vjzrM48bb

[kkarbowiak]

Seems to be due to a static cast to an unsigned type:

fmt/include/fmt/format.h

Line 4134 in 365c3fb

static_cast<detail::uint64_or_128_t<T>>(t.value),

[vitaut]

Fixed by #3901.