diff_util: Add the short alias `S` for `--stat` #17068

Job	Run time
Check protos	23s
Check formatting	17s
Build jj-lib without Git support	49s
build (ubuntu-latest)	5m 15s
cargo-deny (advisories)	26s
build (macos-13)	6m 1s
cargo-deny (bans licenses sources)	26s
build (macos-14)	5m 17s
Clippy check	2m 35s
build (windows-latest)	12m 0s
Check that MkDocs can build the docs	9s
build (ubuntu-latest, --all-features)	6m 4s
Check that MkDocs can build the docs with latest Python and uv	14s
	39m 56s

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