update.sh: add instruction to delete shallow file

[michaels-lyft]

I was still getting the following error even after running git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask" fetch --unshallow and saw that the .git/shallow file was not removed after the git command.

Error: homebrew-cask is a shallow clone. To `brew update` first run:
  git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask" fetch --unshallow

This change adds the instruction to delete the shallow file after the repo is unshallowed.

Related PR: #9383

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[gromgit]

Have you actually encountered a situation where git fetch --unshallow fails to complete the unshallow process (including the removal of .git/shallow), yet returns success?

If you have, that's a Git bug, and should be reported as such. Your addition would then also be a bug, since you're nuking shallowed state from a still-shallowed repo.

If not, your addition does nothing useful.

[michaels-lyft]

Have you actually encountered a situation where git fetch --unshallow fails to complete the unshallow process (including the removal of .git/shallow), yet returns success?

If you have, that's a Git bug, and should be reported as such. Your addition would then also be a bug, since you're nuking shallowed state from a still-shallowed repo.

If not, your addition does nothing useful.

Hmm, I tried it again with another repo and git was able to delete the file. Never mind.