diff: add "diff" label globally by outer show_diff/patch() functions #12905

Job	Run time
Build jj-lib without Git support	41s
build (ubuntu-latest)	4m 20s
build (macos-14)	3m 45s
Check protos	38s
build (windows-latest)	8m 9s
build (ubuntu-latest, --all-features)	5m 27s
Check that MkDocs can build the docs	24s
Check that MkDocs can build the docs with Poetry 1.8	22s
Check formatting	17s
cargo-deny (advisories)	24s
cargo-deny (bans licenses sources)	22s
Clippy check	2m 34s
	27m 23s

Provide feedback