pack: add a constant for max hash size

With SHA-256, our maximum hash size is increasing, so we'll need to
increase the size of our buffers when reading.  However, we'd like to
avoid just hard-coding the value 32, so let's create a constant we can
update easily should we need to change things in the future.

Author: bk2204

pack/index.go

@@ -2,10 +2,13 @@ package pack
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"fmt"
 	"io"
 )
 
+const maxHashSize = sha256.Size
+
 // Index stores information about the location of objects in a corresponding
 // packfile.
 type Index struct {