build

FR: Add new revset `builtin_immutable_heads()` #13241

Job	Run time
Build jj-lib without Git support	52s
build (ubuntu-latest)	4m 27s
Check protos	26s
Check that MkDocs can build the docs	28s
build (macos-14)	4m 30s
build (windows-latest)	9m 19s
Check formatting	18s
build (ubuntu-latest, --all-features)	5m 43s
cargo-deny (advisories)	25s
cargo-deny (bans licenses sources)	24s
Clippy check	2m 43s
Check that MkDocs can build the docs with Poetry 1.8	30s
	30m 5s

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