completion: teach jj about aliases #15999

Job	Run time
build (ubuntu-latest)	4m 50s
Build jj-lib without Git support	43s
build (macos-13)	8m 51s
Check protos	42s
build (macos-14)	4m 32s
Clippy check	2m 29s
build (windows-latest)	11m 20s
Check that MkDocs can build the docs	24s
build (ubuntu-latest, --all-features)	5m 55s
Check that MkDocs can build the docs with Poetry 1.8	20s
Check formatting	18s
cargo-deny (advisories)	25s
cargo-deny (bans licenses sources)	23s
	41m 12s

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