Jupyter-book: Binder: Spec is not of the form "user/repo/ref"

[cedeerwe]

Describe the bug
Pressing the Binder or the Thebelab button both result in the same error, which is

Binder: Spec is not of the form "user/repo/ref", provided: "cedeerwe/test_book". Did you mean "cedeerwe/test_book/master"?

To Reproduce

1. Build a book using the standard jb build test-book.
2. Deploy the book to gh-pages using the command from the docs ghp-import -n -p -f test-book/_build/html. The page is rendered correctly, as seen here.
3. Press the Thebelab or Binder buttons. Binder results in a 400 error, while Thebelab gets stuck at waiting for kernel with the error described above in the console.

Expected behavior
Thebelab and binder would be launched with the correct user/repo/ref form.

Link to repo
https://github.com/cedeerwe/test_book

Environment (please complete the following information):

• Python 3.6.9
• Output of jupyter-book --version

Jupyter Book: 0.7.0
MyST-NB: 0.8.3
Sphinx Book Theme: 0.0.25
MyST-Parser: 0.8.2
Jupyter-Cache: 0.2.2

Additional context
I remember Thebelab having an issue already about a month ago, but I am not sure whether this was the same problem.

[choldgraf]

Ah good catch - I think this is because the default branch isn't being picked up properly and sent to the Binder link. Will investigate!

[choldgraf]

OK, should be fixed and I released a new version of the sphinx-book-theme, so give a shot upgrading that in a few minutes and see if this now works for you!

[cedeerwe]

Wow, that was a super quick fix. Thanks a lot!