build

merged_tree: remove `TreeDiffEntry::source` #13521

Job	Run time
Build jj-lib without Git support	42s
build (ubuntu-latest)	4m 27s
Check protos	31s
cargo-deny (advisories)	22s
build (macos-13)	5m 20s
Clippy check	2m 29s
cargo-deny (bans licenses sources)	21s
build (macos-14)	3m 23s
Check that MkDocs can build the docs	19s
build (windows-latest)	8m 20s
Check that MkDocs can build the docs with Poetry 1.8	22s
build (ubuntu-latest, --all-features)	5m 24s
Check formatting	15s
	32m 15s

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