Set receive.shallowUpdate on to allow pushing from shallow clones

Just that, I don't think it can be done by the user now, as it implies modifying the remote .git/config. See these links for some additional context:

http://stackoverflow.com/questions/33085672/shallow-update-not-allowed-git-1-9

http://stackoverflow.com/questions/33087119/setting-an-option-in-a-remote-git-repository

Note: of course, this is for new empty repositories, not for ones imported from elsewhere.