handle discovery of split worktrees #3453

Job	Run time
test-32bit	59s
pure-rust-build	3m 38s
test-fast (windows-latest)	13m 31s
test-fast (macos-latest)	4m 34s
cargo-deny (advisories)	40s
cargo-deny (bans licenses sources)	31s
installation (win-msvc)	7m 40s
test-fast (ubuntu-latest)	7m 26s
installation (win-gnu)	8m 37s
installation (win32-msvc)	8m 0s
lint	7m 12s
installation (win32-gnu)	9m 5s
test	30m 28s
WebAssembly (wasm32-unknown-unknown)	1m 30s
WebAssembly (wasm32-wasi)	52s
	1h 44m 43s

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