Hi,
The zsh completion file overrides the user-commands zstyle value. IMHO is a bad practice as it is going to cause confusion to the end user when using conflicting packages (like git-extras) or local commands. I would suggest that you fix the use of the command with something like the following. I can create a pull request if you prefer.
zstyle -g existing_commands ':completion:*:*:git:*' user-commands
zstyle ':completion:*:*:git:*' user-commands $existing_commands flow:'provide high-level repository operations'
Thanks
Regards