util: cast memoryview to bytes

Python 3 uses readinto() instead of read() in places. And
taking a slice of the buffer passed to readinto() will produce
a memoryview. _writedata() then gets confused when testing for
`b'\n' in data` because memoryview is an iterable over ints
instead of 1 character bytes.

We work around by casting a memoryview to bytes.

Differential Revision: https://phab.mercurial-scm.org/D5704

mercurial/util.py

@@ -789,6 +789,12 b' class fileobjectobserver(baseproxyobserv'
                                                       res))
 
         data = dest[0:res] if res is not None else b''
+
+        # _writedata() uses "in" operator and is confused by memoryview because
+        # characters are ints on Python 3.
+        if isinstance(data, memoryview):
+            data = data.tobytes()
+
         self._writedata(data)
 
     def write(self, res, data):