Just prefix your install command with HOMEBREW_NO_AUTO_UPDATE=1, like this:

HOMEBREW_NO_AUTO_UPDATE=1 brew install somepackage

Source: brew manpage

You need to reset your Cellar git repo.

cd `brew --cellar`
git reset --hard HEAD
brew upgrade <formula>

That should do it!

So I found a solution. Unlucky me stumbled upon this one I guess.

Used the same solution:

cd $(brew --repo) && git fetch && git reset --hard origin/master && brew update

Update 10/11/2020 to reflect the latest brew changes.

Brew already provide a command to uninstall itself (this will remove everything you installed with Homebrew):

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall.sh)"

If you failed to run this command due to permission (like run as second user), run again with sudo

Then you can install again:

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

It could be downloading something large and taking a long time. Try running the following so you can see the download progress:

brew update --debug --verbose

Did you run brew update-reset && brew update? Can you also please provide the output of brew config and brew doctor --verbose?

Sorry for the trouble here. There was a bug that affected the update script for some users if you updated between 26th June 14:49 UTC and 27th June 16:57 UTC that would only manifest itself the next time you performed an update, even if outside that window. The bug also only affected those with bleeding edge updates enabled (i.e. with developer mode enabled - brew developer).

The easiest fix is brew update-reset.