The homebrew-core repository contains all of the default formulas for Homebrew.
It's a huge repo, and if you browse it through the GitHub web interface you can run into errors like this one:
/~https://github.com/Homebrew/homebrew-core/commits/master/Formula/libspatialite.rb
Refresh the page to try again, or view this history locally using the following command:
git log master -- Formula/libspatialite.rb
It turns out there's a full checkout of the repo (including history) in this folder on your computer already:
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
So you can browse the history for that file locally like so:
cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
git log master -- Formula/libspatialite.rb