path seems not to be inherited when spawning git

Hi, first up, thanks for this awesome idea -- I only found patch-package a couple of days ago and I'm already in love! Simple but so useful. I'm looking forward to removing lots of forked dependencies from my repo in coming days!

I'm on a Mac, and when I first asked patch-package to create a patch, I got a strange error:
> `xcode-select: note: no developer tools were found at '/Applications/Xcode.app', requesting install. Choose an option in the dialog to download the command line developer tools.`

This is the output from /usr/bin/git, which is a small stub on Macs that asks the user to install Xcode. I already have the full git suite installed, and on my $PATH at /usr/local/bin/git, and I use VS Code, so I don't want to install Xcode too.

I found that if I change this line in makePatch.js
> `env: { HOME: tmpRepo.name },`

to
> `env: { ...process.env, HOME: tmpRepo.name },`

patch-package started running the correct version of git, and was able to create a patch.

I think the unchanged line might discard all existing environment variables in the spawned process, including $PATH. But I don't know how to be sure, because I can't spawn 'set' (it's a built-in) and if I spawn a sub-shell like 'bash -c set', it picks up the correct path from .bashrc.

Needless to say, after getting this working, I used patch-package to patch itself :-D

If you think this is not a bug please feel free to ignore me, it may just be something is weird or broken on my system! Thanks again