Add a button to clone branches

To clone a branch manually:

1. Go to the branch you want to clone.
2. Click Edit on any file. Do not make any changes.
3. Make sure you have selected this:

1. Open Chrome Dev Tools.
2. Inspect the Propose file change button.
3. Remove the disabled attribute.
4. Click Propose file change.

🎉 In other words, it makes a spin off from your branch and makes an empty commit (I don’t know of any solution that wouldn’t create an empty commit).

[fregante]

Explained in #2500: it doesn’t move PRs

[fregante]

You can already clone it:

1. Visit branch
2. Open branch selector
3. Type new branch name
4. Click create