meson subprojects: add status and gitignore commands #16812

Job	Run time
Arch Linux	20m 16s
Arch / PyPy	20m 0s
CUDA (on Arch)	7m 47s
Fedora	19m 51s
Gentoo	22m 57s
Ubuntu Rolling (gcc, g++)	21m 24s
OpenSUSE	16m 17s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 55s
Ubuntu Bionic	14m 54s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	15m 4s
Ubuntu Rolling (clang, clang++)	22m 25s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 36s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 25s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 5s
	3h 58m 56s

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