bundle2.unbundlepart: tell() implementation

Keep track of how many bytes we've returned from read(); fairly straightforward.

mercurial/bundle2.py

@@ -828,6 +828,7 b' class unbundlepart(unpackermixin):'
         self._readheader()
         self._mandatory = None
         self._chunkindex = [] #(payload, file) position tuples for chunk starts
+        self._pos = 0
 
     def _fromheader(self, size):
         """return the next <size> byte from the header"""
@@ -930,8 +931,12 b' class unbundlepart(unpackermixin):'
             data = self._payloadstream.read(size)
         if size is None or len(data) < size:
             self.consumed = True
+        self._pos += len(data)
         return data
 
+    def tell(self):
+        return self._pos
+
 capabilities = {'HG2Y': (),
                 'b2x:listkeys': (),
                 'b2x:pushkey': (),