Add an option to refuse to checkout PRs greater than --checkout-depth

[DerekTBrown]

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

Describe the user story

• As the operator of an Atlantis deployment, I want the option to prevent users from planning/applying PRs older than --checkout-depth to avoid Atlantis performing full clones.

Describe the solution you'd like

• Add an additional --disallow-full-clone option to Atlantis server.
• If enabled and the merge base isn't present in a shallow checkout¹, Atlantis will fail instead of performing a full clone.

Describe the drawbacks of your solution

• This adds additional complexity to Atlantis.

Describe alternatives you've considered

• It is hypothetically possible to implement this as a pre-workflow hook², however, such an implementation would be brittle, as it must independently communicate with SCM. A native feature seems relatively lightweight, and would be highly useful for operators using Atlantis in a monorepo.

Footnotes

1. https://www.runatlantis.io/docs/checkout-strategy#:~:text=If%20the%20merge%20base%20is%20not%20present%2C%20it%20means%20that%20either%20of%20the%20branches%20are%20ahead%20of%20the%20merge%20base%20by%20more%20than%20%2D%2Dcheckout%2Ddepth%20commits.%20In%20this%20case%20full%20repo%20history%20is%20fetched ↩

2. https://www.runatlantis.io/docs/pre-workflow-hooks.html#custom-run-command ↩