lib/scanner, vendor: Fix previous commit

Can't do what I did, as the rolling function is not the same as the
non-rolling one. Instead this uses an improved version of the rolling
adler32 to accomplish the same thing. (PR filed on upstream, so should
be able to use that directly in the future.)

vendor/github.com/chmduquesne/rollinghash/adler32/adler32.go

@@ -3,6 +3,7 @@
 package adler32
 
 import (
+	"hash"
 	vanilla "hash/adler32"
 
 	"github.com/chmduquesne/rollinghash"
@@ -22,6 +23,8 @@ type digest struct {
 	window []byte
 	oldest int
 	n      uint32
+
+	vanilla hash.Hash32
 }
 
 // Reset resets the Hash to its initial state.
@@ -37,7 +40,7 @@ func (d *digest) Reset() {
 // only used to determine which is the oldest element (leaving the
 // window). The calls to Roll() do not recompute the whole checksum.
 func New() rollinghash.Hash32 {
-	return &digest{a: 1, b: 0, window: nil, oldest: 0}
+	return &digest{a: 1, b: 0, window: nil, oldest: 0, vanilla: vanilla.New()}
 }
 
 // Size returns the number of bytes Sum will return.
@@ -53,13 +56,15 @@ func (d *digest) BlockSize() int { return 1 }
 // running hash. It never returns an error.
 func (d *digest) Write(p []byte) (int, error) {
 	// Copy the window
-	d.window = make([]byte, len(p))
+	if len(d.window) != len(p) {
+		d.window = make([]byte, len(p))
+	}
 	copy(d.window, p)
 
 	// Piggy-back on the core implementation
-	h := vanilla.New()
-	h.Write(p)
-	s := h.Sum32()
+	d.vanilla.Reset()
+	d.vanilla.Write(p)
+	s := d.vanilla.Sum32()
 	d.a, d.b = s&0xffff, s>>16
 	d.n = uint32(len(p)) % mod
 	return len(d.window), nil