I implemented this at Expand loading from git repositories to support links to repository root · elm-time/elm-time@6c5d806 · GitHub
You can now use the link to the repository. The loading from git now also works for the URL you find in the address bar on the repository homepage. In this case, Elm Editor uses the HEAD reference from the git repository to determine the default branch.
(Meanwhile I also updated the demo instance to integrate that change)