conflicts: use non-legacy `MergedTreeId` for root commit #12753

Job	Run time
cargo-deny (advisories)	22s
Build jj-lib without Git support	42s
Check protos	35s
build (ubuntu-latest)	4m 15s
cargo-deny (bans licenses sources)	23s
build (macos-14)	3m 25s
Check that MkDocs can build the docs	20s
build (windows-latest)	7m 48s
Clippy check	2m 29s
build (ubuntu-latest, --all-features)	5m 21s
Check that MkDocs can build the docs with Poetry 1.8	25s
Check formatting	14s
	26m 19s

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