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/mailhash/mailhash.go

@@ -34,8 +34,6 @@ type Messages struct {
 	Statuses []Status
 }
 
-var headers = []string{"to", "from", "cc", "date", "subject"}
-
 func (m *Messages) hashMail(path string, info os.FileInfo, err error) error {
 	glog.Infoln("Processing", path)
 	if err != nil {
@@ -58,7 +56,7 @@ func (m *Messages) hashMail(path string, info os.FileInfo, err error) error {
 	}
 	defer r.Close()
 
-	h, err := email.Hash(r, headers)
+	h, err := email.HashReader(r)
 	if err != nil {
 		glog.Errorf("%s not an mail file", path)
 		glog.Infof("%q", err.Error())
@@ -67,7 +65,7 @@ func (m *Messages) hashMail(path string, info os.FileInfo, err error) error {
 	md := email.NewInfo(path)
 	m.Statuses = append(m.Statuses, Status{
 		Path: path,
-		Hash: fmt.Sprintf("%x", h.Sum(nil)),
+		Hash: h,
 		Read: md.Seen,
 	})
 	return nil
@@ -139,14 +137,13 @@ func (m Messages) Reconcile(maildir string) error {
 		}
 		defer r.Close()
 
-		h, err := email.Hash(r, headers)
+		chksum, err := email.HashReader(r)
 		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))
 		md := email.NewInfo(path)
 		s, ok := hashMap[chksum]
 		if !ok {