When you want to update Homebrew, use brew update. After running brew update, you may have outdated formulae installed, which is where brew upgrade is necessary.
$ git -C
Takes an existing commit object, and reuses the log messages and the authorship information (including the timestamp) when creating a commit.
This is the path to the homebrew-core folder on a computer.
Downloads objects and references from another repository.
This is a git fetch option. If the source repository is complete, it converts a shallow repository to a complete one, removing all the limitations imposed by shallow repositories.
Updates the Homebrew