Add option to ignore nested git repositories #2604

Job	Run time
rustfmt	8s
docs	11s
test (pinned, ubuntu-latest, 1.74.0)	1m 23s
test (stable, ubuntu-latest, stable)	59s
test (beta, ubuntu-latest, beta)	1m 2s
test (nightly, ubuntu-latest, nightly)	1m 4s
test (stable-musl, ubuntu-latest, stable, x86_64-unknown-linux-musl)	3m 30s
test (stable-x86, ubuntu-latest, stable, i686-unknown-linux-gnu)	1m 20s
test (stable-aarch64, ubuntu-latest, stable, aarch64-unknown-linux-gnu)	3m 22s
test (stable-arm-gnueabihf, ubuntu-latest, stable, armv7-unknown-linux-gnueabihf)	3m 14s
test (stable-arm-musleabihf, ubuntu-latest, stable, armv7-unknown-linux-musleabihf)	4m 7s
test (stable-arm-musleabi, ubuntu-latest, stable, armv7-unknown-linux-musleabi)	3m 54s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	3m 23s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	2m 57s
test (macos, macos-latest, nightly)	1m 11s
test (win-msvc, windows-2022, nightly)	2m 5s
test (win-gnu, windows-2022, nightly-x86_64-gnu)	2m 34s
	36m 24s

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