diff: handle "matching" regions of different lengths, minor cleanup #14638

Job	Run time
Build jj-lib without Git support	44s
Check protos	24s
build (ubuntu-latest)	4m 21s
build (macos-13)	11m 50s
Check that MkDocs can build the docs	21s
Check that MkDocs can build the docs with Poetry 1.8	24s
build (macos-14)	4m 13s
build (windows-latest)	10m 5s
build (ubuntu-latest, --all-features)	5m 22s
Check formatting	16s
cargo-deny (advisories)	22s
Clippy check	2m 31s
cargo-deny (bans licenses sources)	23s
	41m 16s

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