Use case: You currenty use GitHub or another git service via ssh, and have a defualt user configured in your ~/.gitconfig and uses your ~/.ssh/id_rsa key for connecting. So far so good.
Now, you want to use a separate user for certain repositories, for some reason. Here it is an explanation on how to do this.
First, add something like this to ~/.ssh/config:
Host github-otheruser IdentityFile ~/.ssh/id_rsa-otheruser User git HostName github.com Port 22
The, just clone the repository and configure user in one command:
git clone github-otheruser:otheruser/myrepo \ --config=user.name='Other User' [email protected]