Warn the user if they are not in a git repo - bail out gracefully

[mikelinhas]

Description of the problem

When the tool is used outside a repo, it breaks and prints out the stdout from the caught error.

Solution

Before running the create commit command (for example), first check if the user is in a git repository.

Alternatives

This could be done before asking the user for input, since we know the tool will fail outside a repo.

Additional context

I saw there is a Typescript migration proposal for this repo. I programmed it with Flow, or at least I think I did, since I have no prior knowledge of Flow.

Validations

• [X] Follow our Code of Conduct
• [X] Read the Contributing Guide.
• [X] Check that there isn't already an issue requesting the same feature.