original brew update Error needs to run git fetch --unshallow, but got another git fatal

I dug a little after trying to deal with the git repo, but the simplest thing that fixed it for me was untapping and then retapping the casks repo

brew untap homebrew/cask
brew tap homebrew/cask

that put everything back in order.

I haven't RTFM'd lately, but I'm guessing this has something to do with how casks seem to be moving into the main homebrew repo (or at least that's how it seems as a naive user).

I tried to run: $ git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch

then: $ git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow

brew update seems OK now.