config: migrate off `config.merge()` #16859

Job	Run time
build (ubuntu-latest)	5m 1s
build (macos-13)	8m 33s
build (macos-14)	3m 55s
build (windows-latest)	10m 18s
build (ubuntu-latest, --all-features)	6m 3s
Build jj-lib without Git support	48s
cargo-deny (advisories)	25s
cargo-deny (bans licenses sources)	28s
Check protos	29s
Clippy check	2m 58s
Check that MkDocs can build the docs	11s
Check that MkDocs can build the docs with latest Python and uv	12s
Check formatting	17s
	39m 38s

Provide feedback