Add support for git subtree workflow (commands)

**Is your feature request related to a problem? Please describe.**
I started using git-subtree on my repos, to integrate small repos into bigger ones, and lazygit doesn't offer support for them, so I had to go back to typing obscure and verbose commands in the console. Definitively a step backward in my daily work.

**Describe the solution you'd like**
It would be great if lazygit added commands or support for git subtrees.

Thanks in advance for considering this.
Cheers.
Max.