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Describe the bug

The / in branch name is not escaped properly. e.g.,

https://nbviewer.org/github/sighingnow/v6d/tree/main/

Selecting a branch with / in name will cause 404, but replace the / in the branch name part with / in the URL will open this branch correctly.

To Reproduce

Steps to reproduce the behavior:

1. Go to 'https://nbviewer.org/github/sighingnow/v6d/tree/main/'
2. Click on 'main' to select a branch, then choose dev/kubernetes
3. Scroll down to '....'
4. See error

Expected behavior

The / in branch name should be escaped.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.