[Feature] Create a "git init" sub-generator

This is the same idea as #6440

I would like to have a small sub-generator for Git:

• jhipster git-init would initialize a Git repository, add all files to it, and commit with message “Initial application generation by JHipster” (like https://start.jhipster.tech does today)
• This would run by default after each generation, but could be disabled with a flag

I’m also wondering if we could have the same after an entity is generated, but I’m less sure it’s a good idea.

[deepu105]

so you ok to do it directly in the app generator without a sub gen?

[jdubois]

So I removed the logs, but I’m not sure it’s good for error handling… Anyway, as long as Git is installed, all commands should work (if you can do a git init, I don’t understand how a git add .would fail)