cli: diff: add `--exclude` flag #16610

Job	Run time
build (ubuntu-latest)	5m 1s
Build jj-lib without Git support	47s
cargo-deny (advisories)	24s
build (macos-13)	6m 1s
cargo-deny (bans licenses sources)	23s
build (macos-14)	3m 51s
Check protos	24s
build (windows-latest)	10m 30s
build (ubuntu-latest, --all-features)	6m 0s
Check that MkDocs can build the docs	10s
Check that MkDocs can build the docs with latest Python and uv	14s
Check formatting	18s
Clippy check	2m 36s
	36m 39s

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