More intelligent git caching

[DerekTBrown]

• I'd be willing to implement this feature (contributing guide)

Describe the user story

• As a Atlantis user, I want Atlantis to cache upstream commits so that I can use Atlantis in my mono-repo, without the slowdown associated cloning the entire repository every time a plan occurs.

Describe the solution you'd like

• Atlantis could use git worktree to check out the same repository at multiple commits, reducing storage usage and functionally caching the repo between plans.

Describe the drawbacks of your solution

• This would make Atlantis somewhat stateful, since the Atlantis pod would have cached data.
• This would add complexity to Atlantis.

Describe alternatives you've considered

• The best Git performance tool at the moment is --checkout-depth. While this is a decent optimization, in a high-volume Atlantis environment, there is still a lot of needless git traffic.