With "export" remote-helper protocol,

 (1) a push that tries to update a remote ref whose name is
     different from the pushing side does not work yet, and

 (2) the helper may not know how to do --dry-run

Detect such problematic cases and disable them for now.

* fc/transport-helper-no-refspec:
  transport-helper: check if the dry-run is supported
  transport-helper: barf when user tries old:new