Add helper to print message with hash as a header.

Broke out email.go's Hash() in to multiple functions to make this easier, and
update tools that used it.

cmd/md2pq/md2pq.go

@@ -29,9 +29,6 @@ var (
 	skipFiles = flag.String("skip", "maildirfolder,log,msgid.cache,razor-agent.log",
 		"comma separated files to skip")
 
-	// Hashed over fields from each message.
-	headers = []string{"to", "from", "cc", "date", "subject", "message-id"}
-
 	total  = expvar.NewInt("bytes-parsed")
 	cnt    = expvar.NewInt("messages-parsed")
 	dupCnt = expvar.NewInt("duplicates-found")
@@ -101,14 +98,13 @@ func Load(db *sql.DB, uid int, root string, skip *set.StringSet) error {
 			}
 
 			hdr_size := bytes.Index(b, CRCR)
-			h, err := email.Hash(bytes.NewReader(b), headers)
+			chksum, err := email.HashReader(bytes.NewReader(b))
 			if err != nil {
 				glog.Errorf("%s not an mail file", path)
 				glog.Infof("%q", err.Error())
 				return nil
 			}
 
-			chksum := fmt.Sprintf("%x", h.Sum(nil))
 			if _, err := hstmt.Exec(chksum, path); err != nil {
 				return err
 			}