Where is "the line" for copyleft licenses

Where exactly is "the line" between needed to copy a copyleft license and not? For example, I am writing a non GNU GPL program that uses git. I do not mean that I use git to write it, I mean, git is actually a vital part of what the program does. The code itself calls git.

Can I do this without needing to copyleft? Can I distribute git with my program? Or do I have to require users to install it separately?

I know that a "fork" of git would be copyleft and just using git to write code does not. What about in-between?