If `/usr/local` is the prefix but not the repository (so bottles can be
used) then running `/usr/local/bin/brew` works great but
`$HOMEBREW_REPOSITORY/bin/brew` assumes the prefix is
`$HOMEBREW_REPOSITORY`. This is a pain when doing work on the Homebrew
repository and having e.g. `bin` in your `$PATH`. Improve this
behaviour so `bin/brew` knows how to handle this situation.