Avoid evaluating large revsets just to check if they're immutable #6809

Job	Run time
check (rustfmt)	22s
check (protos)	48s
nix flake (macos-14)	8m 24s
nix flake (ubuntu-24.04-arm)	6m 51s
check (clippy)	1m 38s
nix flake (ubuntu-24.04)	7m 37s
test (linux-x86_64-gnu)	6m 9s
test (macos-aarch64)	6m 11s
test (macos-x86_64)	10m 16s
test (windows-aarch64)	16m 51s
build (no git)	1m 29s
test (linux-aarch64-gnu)	5m 42s
check (cargo-deny, advisories)	25s
check (latest mkdocs, optional)	7m 22s
test (windows-x86_64)	18m 23s
check (codespell)	10s
check (mkdocs)	14s
check (cargo-deny, bans)	26s
check (Cargo.lock dependency count)	6s
check (cargo-deny, licenses)	29s
check (doctests)	3m 0s
check (cargo-deny, sources)	23s
check (zizmor)	23s
required checks (merge queue)	0s
	1h 43m 39s

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