surf: Add `stats` property to each `DiffContent::Plain` #715

Job	Run time
build-windows	4m 33s
cargo-deny (advisories)	27s
build-macos	1m 28s
cargo-deny (bans licenses sources)	24s
build-linux (stable)	40s
docs	29s
build-linux (nightly)	59s
fmt	25s
lint	33s
test-linux (stable)	1m 18s
test-linux (nightly)	1m 15s
test-macos	1m 20s
	13m 51s

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