completion: suggest file paths incrementally #16765

Job	Run time
build (ubuntu-latest)	5m 15s
Build jj-lib without Git support	45s
build (macos-13)	6m 6s
build (macos-14)	3m 45s
cargo-deny (advisories)	22s
build (windows-latest)	10m 2s
cargo-deny (bans licenses sources)	25s
build (ubuntu-latest, --all-features)	6m 6s
Clippy check	2m 33s
Check protos	31s
Check that MkDocs can build the docs	10s
Check that MkDocs can build the docs with latest Python and uv	10s
Check formatting	17s
	36m 27s

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