[oh-my-zsh] Can't update: not a git repository.

After updating macOS from version 12 to version 13 I got the following error/warning when opening iTerm:

user@host: [oh-my-zsh] Can't update: not a git repository.

This actually has nothing to do with a git repository but it happens due to the missing command line developer tools. So to make the error/warning go away just install command line developer tools and then you can update ohmyzsh and the error/warning will be gone.

user@host: ~|⇒  xcode-select --install
user@host: ~|⇒  omz update
Updating Oh My Zsh
Hooray! Oh My Zsh has been updated!