Use different ssh keys for different github repos (per-url sshCommand)

Hi.

I have two github accounts, one is for my organization and I want git to automatically choose the correct ssh `IdentityFile` based on the clone URL:

    git@github.com:other/publicrepo.git
       ~/.ssh/id_rsa
    git@github.com:theorganization/privaterepo.git
       ~/.ssh/id_rsa.theorganization

Unfortunately, both URLs have same host name, therefore I can't configure this in the ssh client config. I could create a host alias there, but sometimes somebody else gives me the github URL and I want it to work out of the box.

I thought I could add a per-URL `core` section similar to `user` and `http`, but this section is ignored by git (2.18):

    [core "git@github.com:theorganization"]
        sshCommand = /bin/false
        #sshCommand = ssh -i ~/.ssh/id_rsa.theorganization

I thought of writing a wrapper script to deduce the key from the arguments:

    git@github.com git-upload-pack '/theorganization/privaterepo.git'

Is this the only option?