Fixed digamma unit test #2438

Job	Run time
macosx (clang, x86_osx, -mavx)	1h 24m 24s
externals (clang, x86, -msse2)	44m 4s
externals (gcc, x86, -msse2)	56m 13s
no_pch (clang, x86, -msse2)	57m 22s
no_pch (gcc, x86, -msse2)	1h 3m 0s
nosimd (clang, x86, -DEVE_NO_SIMD)	38m 35s
nosimd (gcc, x86, -DEVE_NO_SIMD)	50m 59s
riscv (clang, riscv, -mrvv-vector-bits=128)	33m 37s
x86-clang (clang, x86, -msse4.2)	17m 46s
x86-clang (clang, x86, -mavx)	23m 51s
x86-clang (clang, x86, -mavx2)	22m 57s
arm (clang, aarch64, -Wno-psabi)	33m 37s
arm (clang, arm, -Wno-psabi)	33m 56s
other-arch (gcc, sve256, -Wno-psabi)	1h 9m 38s
other-arch (gcc, sve512, -Wno-psabi)	2h 37m 36s
other-arch (gcc, ppc64, -Wno-psabi)	42m 43s
avx512 (clang, x86, -march=skylake-avx512)	37m 11s
avx512 (gcc, x86, -march=skylake-avx512)	1h 2m 39s
x86-gcc (gcc, x86, -msse4.2)	15m 55s
x86-gcc (gcc, x86, -mavx)	23m 16s
x86-gcc (gcc, x86, -mavx2)	22m 49s
other-cases (clang_libcpp, x86, -msse2)	43m 47s
other-cases (clang, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	54m 6s
other-cases (gcc, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	1h 13m 5s
	19h 23m 6s

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