compress/flate: fix a typo, improve compression rate by 3-4%.

R=rsc
CC=golang-dev
https://golang.org/cl/5556077

src/pkg/compress/flate/deflate.go

@@ -112,7 +112,7 @@ func (d *compressor) fillDeflate(b []byte) int {
 			d.hashHead[i] = v
 		}
 		for i, h := range d.hashPrev {
-			v := -h - windowSize
+			v := h - windowSize
 			if v < -1 {
 				v = -1
 			}

src/pkg/compress/flate/deflate_test.go

@@ -291,7 +291,7 @@ var deflateInflateStringTests = []deflateInflateStringTest{
 	{
 		"../testdata/Mark.Twain-Tom.Sawyer.txt",
 		"Mark.Twain-Tom.Sawyer",
-		[...]int{416188, 191483, 185232, 179560, 175233, 171263, 169908, 169758, 169712, 169712},
+		[...]int{407330, 187598, 180361, 172974, 169160, 163476, 160936, 160506, 160295, 160295},
 	},
 }