revset: add `coalesce(revsets...)` #14987

Job	Run time
Check protos	30s
build (ubuntu-latest)	4m 31s
build (macos-13)	10m 41s
Build jj-lib without Git support	44s
cargo-deny (advisories)	22s
Clippy check	2m 30s
build (macos-14)	3m 51s
cargo-deny (bans licenses sources)	32s
Check that MkDocs can build the docs	22s
Check that MkDocs can build the docs with Poetry 1.8	22s
build (windows-latest)	9m 53s
Check formatting	18s
build (ubuntu-latest, --all-features)	5m 41s
	40m 17s

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