Add --log-group option to opt-out/force-enable log grouping #2596

Job	Run time
miri	1m 5s
test (stable)	1m 48s
test (beta)	1m 50s
test (nightly)	1m 49s
test (nightly, macos-11)	1m 44s
test (nightly, windows-2019)	1m 41s
test (1.26)	1m 48s
test (1.30)	1m 48s
test (1.31)	1m 48s
test (1.36)	1m 48s
test (1.39)	1m 48s
test (1.41)	1m 47s
build (aarch64-unknown-linux-gnu)	1m 48s
build (aarch64-unknown-linux-musl)	1m 45s
build (aarch64-apple-darwin)	1m 45s
build (aarch64-pc-windows-msvc)	1m 42s
build (x86_64-unknown-linux-gnu)	56s
build (x86_64-unknown-linux-musl)	1m 15s
build (x86_64-apple-darwin)	1m 44s
build (x86_64-pc-windows-msvc)	1m 40s
build (x86_64-unknown-freebsd)	1m 50s
msrv / msrv	1m 46s
deny / deny	17s
tidy / tidy	1m 2s
	38m 14s

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