Add support for git subtree workflow (commands)

[maxborn]

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.