Use reverse logic of incrementing num parts requested #3067

Job	Run time
clang-sanitizers (,thread)	14m 7s
linux-compiler-compat (clang-3)	14m 7s
linux-compat (manylinux1-x64)	14m 5s
clang-sanitizers (,address,undefined)	14m 7s
linux-shared-libs	14m 6s
linux-compiler-compat (clang-6)	14m 6s
linux-compat (manylinux1-x86)	14m 6s
linux-compiler-compat (clang-8)	14m 5s
linux-compat (manylinux2014-x64)	14m 4s
linux-compat (manylinux2014-x86)	14m 5s
byo-crypto	2m 47s
linux-compiler-compat (clang-9)	14m 3s
linux-compat (al2-x64)	14m 7s
linux-compiler-compat (clang-10)	14m 4s
linux-compiler-compat (clang-11)	14m 11s
windows	13m 54s
windows-vc14 (x86)	13m 51s
linux-compiler-compat (gcc-4.8)	14m 4s
windows-vc14 (x64)	13m 56s
linux-compiler-compat (gcc-5)	14m 5s
linux-compiler-compat (gcc-6)	14m 5s
linux-compiler-compat (gcc-7)	14m 3s
downstream	14m 6s
linux-compiler-compat (gcc-8)	14m 7s
linux-debug	14m 5s
osx	14m 5s
windows-app-verifier	5m 17s
windows-shared-libs	13m 34s
	6h 13m 22s

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