Use node process environment variables as base for overridden git process environment

[steveukx]

Currently when simpleGit is configured with an override for a single environment variable:

const git = simpleGit().env('A', 'aaa');

The git child process will be called with an environment of only that override, whereas the default would be to use the process.environment from the containing node process.

This might be intentional, especially in the case of using an object form:

const git = simpleGit().env({ ...process.env, 'A': 'aaa' });

But to allow for more flexibility and cleaner calling code, there should be a constructor plugin to allow opting in / out of this behaviour, eg:

const git = simpleGit({ env: { inherit: true } }).env('A', 'aaa');

Would cause the git child process to be called with the current state of the process.env object as a base, overridden with the explicitly configured {A: 'aaa'}.