merged_tree: remove `TreeDiffEntry::source` #13527

Job	Run time
Build jj-lib without Git support	44s
Check protos	32s
build (ubuntu-latest)	4m 42s
build (macos-13)	5m 15s
Check that MkDocs can build the docs	22s
build (macos-14)	3m 31s
Clippy check	2m 26s
build (windows-latest)	9m 38s
cargo-deny (advisories)	24s
build (ubuntu-latest, --all-features)	5m 30s
Check that MkDocs can build the docs with Poetry 1.8	23s
Check formatting	15s
cargo-deny (bans licenses sources)	21s
	34m 3s

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