Fix group_digits for negative integers #5909

Job	Run time
build (g++-4.8, Debug, 11)	1m 25s
build (g++-4.8, Release, 11)	2m 37s
build (g++-10, Debug, 11)	1m 21s
build (g++-10, Release, 11)	2m 6s
build (clang++-9, Debug, 11)	1m 21s
build (clang++-9, Release, 11)	2m 16s
build (g++-8, Debug, 14, sudo apt install g++-8)	1m 22s
build (g++-8, Debug, 17, sudo apt install g++-8)	1m 23s
build (g++-9, Debug, 17)	1m 36s
build (g++-10, Debug, 17)	1m 35s
build (g++-11, Debug, 20, sudo apt install g++-11)	2m 48s
build (clang++-8, Debug, 17, -stdlib=libc++, sudo apt install clang-8 libc++-8-dev libc++abi-8-dev)	1m 27s
build (clang++-9, Debug, 17, -DFMT_FUZZ=ON -DFMT_FUZZ_LINKMAIN=ON, sudo apt install clang-9)	1m 35s
build (clang++-11, Debug, 20)	1m 42s
build (clang++-11, Debug, 20, -stdlib=libc++, sudo apt install libc++-11-dev libc++abi-11-dev)	1m 47s
	26m 21s

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