patch: make __repr__() return str

Caught by pytype:

    line 969, in __repr__: Function bytes.join was called with the wrong arguments [wrong-arg-types]
    Expected: (self, iterable: Iterable[bytes])
    Actually passed: (self, iterable: Iterator[str])

Differential Revision: https://phab.mercurial-scm.org/D7682

mercurial/patch.py

@@ -963,7 +963,7 b' class header(object):'
         return self.files()[-1]
 
     def __repr__(self):
-        return b'<header %s>' % (b' '.join(map(repr, self.files())))
+        return '<header %s>' % (' '.join(map(repr, self.files())))
 
     def isnewfile(self):
         return any(self.newfile_re.match(h) for h in self.header)