Merge branch 'fc/transport-helper-no-refspec'

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