merged_tree: minor refactor of TreeDiffStreamImpl #16751

Job	Run time
cargo-deny (advisories)	24s
build (ubuntu-latest)	5m 4s
cargo-deny (bans licenses sources)	21s
Build jj-lib without Git support	46s
build (macos-13)	8m 29s
Check protos	31s
build (macos-14)	5m 37s
Clippy check	2m 33s
Check that MkDocs can build the docs with latest Python and uv	10s
build (windows-latest)	10m 36s
Check that MkDocs can build the docs	15s
build (ubuntu-latest, --all-features)	6m 11s
Check formatting	17s
	41m 14s

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