cd 'brew --prefix' : pas de tel fichier ou répertoire

> cd 'brew --prefix'
-bash: cd: brew --prefix: No such file or directory

Erreurs supplémentaires :

> brew update
fatal: 'origin' does not appear to be a git repository
fatal: The remote end hung up unexpectedly
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

> brew doctor
fatal: 'origin' does not appear to be a git repository
fatal: The remote end hung up unexpectedly
Error: undefined method `[]' for nil:NilClass
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/reporting-bugs
/usr/local/Library/Homebrew/cmd/doctor.rb:840:in `check_for_outdated_homebrew'
/usr/local/Library/Homebrew/extend/pathname.rb:310:in `cd'
/usr/local/Library/Homebrew/extend/pathname.rb:310:in `chdir'
/usr/local/Library/Homebrew/extend/pathname.rb:310:in `cd'
/usr/local/Library/Homebrew/cmd/doctor.rb:837:in `check_for_outdated_homebrew'
/usr/local/Library/Homebrew/cmd/doctor.rb:907:in `send'
/usr/local/Library/Homebrew/cmd/doctor.rb:907:in `doctor'
/usr/local/Library/Homebrew/cmd/doctor.rb:906:in `each'
/usr/local/Library/Homebrew/cmd/doctor.rb:906:in `doctor'
/usr/local/bin/brew:83:in `send'
/usr/local/bin/brew:83

Une idée de ce qui se passe ici ?