bundle2: make header reading optional

The `readbundle` function will consume the 4 first bytes to dispatch between
various unbundler. We introduce a way to inform `unbundle20` that the header
has been read and it can be trusted.

mercurial/bundle2.py

@@ -399,9 +399,11 class unbundle20(unpackermixin):
 
     (this will eventually yield parts)"""
 
-    def __init__(self, ui, fp):
+    def __init__(self, ui, fp, header=None):
+        """If header is specified, we do not read it out of the stream."""
         self.ui = ui
         super(unbundle20, self).__init__(fp)
+        if header is None:
         header = self._readexact(4)
         magic, version = header[0:2], header[2:4]
         if magic != 'HG':