[git] support for `cz` alias with commitizen

[brendanfalk]

Sanity checks

• [X] My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• [X] I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue
• [ ] I would like to work on this.

What CLI tool does this relate to?

git

Which statement makes the most sense?

This completion spec isn't complete

Issue Details

https://github.com/commitizen/cz-cli and https://github.com/cajacko/commit both allow users to run git cz but Fig's git completion spec doesn't support this.

The solution is likely to add a check in our alias generator OR create a hidden subcommand called cz. We won't create an unhidden subcommand as default for everyone as git is such a commonly used spec, it would be incorrect to have cz for the vast majority of users who don't have it.