bundle2: let readbundle return unbundle20

The `readbundle` function can now recognize a bundle2 stream and return the
appropriate unbundler. This is required for proper bundle2 support over the
wire.

mercurial/exchange.py

@@ -32,6 +32,8 b' def readbundle(ui, fh, fname, vfs=None):'
         if alg is None:
             alg = changegroup.readexactly(fh, 2)
         return changegroup.unbundle10(fh, alg)
+    elif version == '20':
+        return bundle2.unbundle20(ui, fh, header=magic + version)
     else:
         raise util.Abort(_('%s: unknown bundle version %s') % (fname, version))