revset: add `coalesce(revsets...)` #331

Job	Run time
Build binary artifacts (linux-x86_64-musl)	4m 59s
Build binary artifacts (linux-x86_64-gnu)	5m 11s
Build binary artifacts (linux-aarch64-musl)	5m 13s
Build binary artifacts (linux-aarch64-gnu)	5m 8s
Build binary artifacts (macos-x86_64)	7m 18s
Build binary artifacts (macos-aarch64)	5m 42s
Build binary artifacts (win-x86_64)	8m 19s
	41m 50s

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