`default-branch-button` appears on the default branch #2799

Replaces #2728

1. Visit https://github.com/sindresorhus/refined-github/commit/bf5dce817b31cea8c4d1535759adc81cc194d305
2. Refresh the page
3. Click the "Code" tab

This unnecessary "default branch button" might appear:

<img width="171" alt="unnecessary" src="https://user-images.githubusercontent.com/1402241/74733843-264f9100-5288-11ea-8431-d4c32f6ef824.png">