adamv/homebrew — comment on commit 81898ad1f4bb0b9ded821b35484b39e9ef95a145
File: Library/Homebrew/cmd/install.rb line 17
By mistydemeo:

In 3da76d0f37318c2beafae326bbc029ec4a36047e

This was when `brew upgrade` was added, intended to prevent formulae from installing overtop of themselves as they used to.