build

merged_tree: remove `TreeDiffEntry::source` #13529

Job	Run time
Build jj-lib without Git support	45s
Check that MkDocs can build the docs	26s
Check protos	25s
build (ubuntu-latest)	4m 28s
Check that MkDocs can build the docs with Poetry 1.8	26s
Check formatting	16s
build (macos-13)	8m 11s
cargo-deny (advisories)	23s
build (macos-14)	3m 25s
Clippy check	2m 31s
cargo-deny (bans licenses sources)	22s
build (windows-latest)	9m 4s
build (ubuntu-latest, --all-features)	5m 28s
	36m 10s

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