use git to check that you're in a Git repository

[shiftkey]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

I was running v5.2.4 and electron-forge init to scaffold up a test project within an existing repository, and because I was running it within a subdirectory (rather than at the root of a repository) it didn't see the root .git directory, and instead initialized a Git repository inside a Git repository.

Not sure how many encounter this, but after remembering to cleanup this new .git directory a second time I got caremad enough to look at the code.

I added some tests to cover the scenarios that I think are important here (including the failing case I found), and ported init-git to use git itself to see whether you're in a repository.

Let me know if there's any extra work you'd like me to do while I'm in here, or if there's stuff I've indirectly affected.

[MarshallOfSound]

Seems legit and for some reason I feel as though I should trust You + Git 😆

[MarshallOfSound]

Docs failed because PR came from fork without token cc @malept