Implement the global -v --verbose flag as an environment variable called DEBUG

In T322286#8401802, @dcaro wrote:

The way git works, is that it does not have a verbose/debug flag on the main command, but enables debugging with env vars, and the rest of discovered subcommands each implement it however they prefer.

I think that we might want to do so for all, as in, have a --verbose flag in the parent command (option 1), but allow setting it with something like DEBUG=1, and what this flag does, is it also sets the env var if not there when calling the subcommand so any external subcommand can pick it up if implemented there.