surf: Add `stats` property to each `DiffContent::Plain` #716

Job	Run time
build-windows	3m 21s
cargo-deny (advisories)	26s
build-linux (stable)	46s
build-macos	58s
cargo-deny (bans licenses sources)	1m 3s
build-linux (nightly)	1m 6s
docs	28s
fmt	18s
lint	31s
test-macos	1m 35s
test-linux (stable)	1m 2s
test-linux (nightly)	1m 13s
	12m 47s

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