Homebrew fatal: needed a single revision (MacOs Sierra)

Question

Not sure if this problem is related to upgrading to MacOs Sierra, but since that moment this error occurs when I run \'brew update\'

→ brew update
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Answer 1

Nothing really worked for me, I had to resort to uninstalling and reinstalling Brew:

# change to home directory to avoid other errors later
cd ~

# uninstall brew
rm -rf /usr/local/Cellar /usr/local/.git && cd ~ && brew cleanup

 # reinstall brew
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"