completion: teach jj about aliases #16000

Job	Run time
build (ubuntu-latest)	5m 51s
build (macos-13)	8m 8s
build (macos-14)	4m 13s
build (windows-latest)	10m 38s
build (ubuntu-latest, --all-features)	5m 52s
Build jj-lib without Git support	45s
cargo-deny (advisories)	24s
cargo-deny (bans licenses sources)	24s
Check protos	25s
Clippy check	2m 29s
Check that MkDocs can build the docs	21s
Check that MkDocs can build the docs with Poetry 1.8	21s
Check formatting	18s
	40m 9s

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