pack: add a hash member to pack structures

Let's add a hash member to the Packfile, V1, and V2 structures so that
we can keep track of the hash algorithm in use when parsing these
files.

Author: bk2204

pack/index_v1.go

@@ -2,10 +2,13 @@ package pack
 
 import (
 	"encoding/binary"
+	"hash"
 )
 
 // V1 implements IndexVersion for v1 packfiles.
-type V1 struct{}
+type V1 struct {
+	hash hash.Hash
+}
 
 // Name implements IndexVersion.Name by returning the 20 byte SHA-1 object name
 // for the given entry at offset "at" in the v1 index file "idx".

pack/index_v2.go

@@ -2,10 +2,13 @@ package pack
 
 import (
 	"encoding/binary"
+	"hash"
 )
 
 // V2 implements IndexVersion for v2 packfiles.
-type V2 struct{}
+type V2 struct {
+	hash hash.Hash
+}
 
 // Name implements IndexVersion.Name by returning the 20 byte SHA-1 object name
 // for the given entry at offset "at" in the v2 index file "idx".

pack/packfile.go

@@ -3,6 +3,7 @@ package pack
 import (
 	"compress/zlib"
 	"fmt"
+	"hash"
 	"io"
 	"io/ioutil"
 )
@@ -18,6 +19,9 @@ type Packfile struct {
 	// objects in this packfile.
 	idx *Index
 
+	// hash is the hash algorithm used in this pack.
+	hash hash.Hash
+
 	// r is an io.ReaderAt that allows read access to the packfile itself.
 	r io.ReaderAt
 }