GitHub Collapse-Diffs Button per verfehlix

Adds a button to GitHub commit pages to collapse all diffs.

Useful for big commits where a lot of files have changed and you want to get an overview of all diffs before inspecting single ones in detail.

Source: https://github.com/verfehlix/firefox-addon-github-collapse-all-diff

If there are any issues, please report them here: https://github.com/verfehlix/firefox-addon-github-collapse-all-diff/issues