fix: update FileWorkspace logic to use pull request refs only when PR number is specified

[marcus-rev]

what

• Updating mergeToBaseBranch logic to only checkout by pull/{pr.number}/head if there is indeed a pull request number specified as part of the plan/apply.
• This change should mitigate this error:

failed to build command: running git fetch origin pull/0/head:: fatal: couldn't find remote ref pull/0/head

• Updating clone logic to skip using HEAD^2 when using the checkout merge strategy whilst no PR number is specified. If a plan is triggered remotely on main for instance, this leads to 128 errors from git:

will re-clone repo, could not determine if was at correct commit: git rev-parse HEAD^2: exit status 128: fatal: ambiguous argument 'HEAD^2': unknown revision or path not in the working tree.

• Updating logic to skip making a VCS call when there isn't a pull request. This mitigates this error:

updating project PR status%!(EXTRA *github.ErrorResponse=POST https://api.github.com/repos/<your-org>/<your-terraform-repo>/statuses/master: 422 Validation Failed [{Resource:Status Field:sha Code:custom Message:sha must be a 40 character SHA1}])...

why

• This should fix the issue detailed in /api/plan throws 500 error when using GitHub App  #4850
• Per the docs, POSTs to /api/plan and /api/apply can optionally omit the PR parameter. Therefore, we should check to see if it's 0 before pivoting to a pull request based reference checkout.
• We shouldn't make calls to VCS if there is no pull request to update statuses on. This leads to 422 errors that can be avoided.

tests

references

https://github.com/runatlantis/atlantis/blob/6fe0303279839faadb76e94de94d7c250876b336/runatlantis.io/docs/api-endpoints.md#post-apiplan

Should close #4850

[X-Guardian]

Thanks for this @marcus-rev. Can you add a unit test for this change, and confirm in the PR description that you have tested this in your own local environment.