Clarify behavior when parsing "index" lines

The parsed OIDs are usually prefixes, not full hashes, so rename the
fields appropriately. Also note that values that are too long to be
valid OIDs are still accepted by the library.

gitdiff/gitdiff.go

@@ -18,9 +18,9 @@ type File struct {
 	OldMode os.FileMode
 	NewMode os.FileMode
 
-	OldOID string
-	NewOID string
-	Score  int
+	OldOIDPrefix string
+	NewOIDPrefix string
+	Score        int
 
 	Fragments []*Fragment
 }