py3: do not pass a memoryview to bdiff.bdiff()

This doesn't look nice, but I don't know how to make a zero-copy slice of
bytes which is compatible with the buffer protocol.

mercurial/mdiff.py

@@ -30,9 +30,17 b' blocks = bdiff.blocks'
 fixws = bdiff.fixws
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
-textdiff = bdiff.bdiff
+_textdiff = bdiff.bdiff
 splitnewlines = bdiff.splitnewlines
 
+# On Python 3, util.buffer() creates a memoryview, which appears not
+# supporting the buffer protocol
+if pycompat.ispy3:
+    def textdiff(a, b):
+        return _textdiff(bytes(a), bytes(b))
+else:
+    textdiff = _textdiff
+
 class diffopts(object):
     '''context is the number of context lines
     text treats all files as text