Fix `irmin push`
Fix `irmin push`

* Irmin push is not an exact symetric of Irmin pull, so handle the case
  more gracefully
* The binary format of the remote and the local store might not be the
  same so convert to JSON as an intermediate value. That's not completely
  satisfactory but that's better than failing with weird binary tag error.

mirage/irmin,mirage/irmin,samoht/irmin