the Tree/tags/branch drop-down list shows a single branch "master".
Choosing it results in a 404 failure with the following string in the address bar,
https://github.com/Homebrew/brew/tree/master/["Library", "Homebrew", "utils"]
I have moved this post to the How to use Git and GitHub board as I think you'll find better answers here.
To answer your question, when I use the dropdown to select the Master branch, I get taken to this URL which seems to work: https://github.com/Homebrew/brew/tree/master/Library/Homebrew/utils
Are you still experiencing this problem?