py3: add __bytes__() for mq.patchheader and make sure __str__ returns str

Before this patch, __str__() function for mq.patchheader class return bytes
which is not str on Python 3. So let's move that logic to __bytes__() and for
__str__() convert the return value of __bytes__() to str.

Differential Revision: https://phab.mercurial-scm.org/D2042

hgext/mq.py

@@ -396,12 +396,14 b' class patchheader(object):'
                 self.comments.append('')
             self.comments.append(message)
 
-    def __str__(self):
+    def __bytes__(self):
         s = '\n'.join(self.comments).rstrip()
         if not s:
             return ''
         return s + '\n\n'
 
+    __str__ = encoding.strmethod(__bytes__)
+
     def _delmsg(self):
         '''Remove existing message, keeping the rest of the comments fields.
         If comments contains 'subject: ', message will prepend