Now this was annoying. I tried a git clone ssh://firstname.lastname@example.org:/path/to/repository and it failed with
Initialized empty Git repository in /current/path/repository/.git/ ssh: Could not resolve hostname my.host:: Name or service not known fatal: The remote end hung up unexpectedly
The solution was rather easy, see man 1 git-clone. A valid git clone command looks like this:
git clone ssh://email@example.com/absolute/path/to/repo.git/
An alternative notation that allows for relative paths is
git clone firstname.lastname@example.org:relative/path/to/repo.git/
The problem with the command I used initially was that I tried to use an scp-like syntax.