util: fix the signature of observedbufferedinputpipe._fillbuffer()

Flagged by PyCharm, since it didn't match the signature of the method being
overridden.  The default value in the superclass is also `_chunksize`, and I
suspect that the amount read from `osread` should be limited to what is passed
in.  Only one caller (`bufferedinputpipe.unbufferedread()`) passes this
argument, and it passes the max of `_chunksize` and whatever it was passed.

mercurial/util.py

@@ -642,12 +642,12 b' class observedbufferedinputpipe(buffered'
     ``read()`` and ``readline()``.
     """
 
-    def _fillbuffer(self):
-        res = super(observedbufferedinputpipe, self)._fillbuffer()
+    def _fillbuffer(self, size=_chunksize):
+        res = super(observedbufferedinputpipe, self)._fillbuffer(size=size)
 
         fn = getattr(self._input._observer, 'osread', None)
         if fn:
-            fn(res, _chunksize)
+            fn(res, size)
 
         return res