Default branch should be the default HEAD branch instead of .*

[nitrocode]

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

Describe the user story

PRs can target feature branches or any branch by default in order to run a terraform plan and apply. This is dangerous because it can circumvent codeowners and apply things outside of normal practices which then results in competing PRs with unintuitive plans.

Imagine this

1. Bob puts in a pr1 targeting main for dir1 using branch s3-fix
2. Bob puts in a pr2 targeting s3-fix for dir1 using branch lambda-fix
3. Bob gets approval by a non codeowner in pr2, atlantis applies pr2 lambda-fix which merges into pr1 s3-fix
4. Sue puts in pr3 targeting main making a change to the same dir1 as the others. She notices there are lambda reversions in the plan which revert pr2s applied changes
5. Sue now needs to investigate why these changes appear. She may assume clickops or some other reason. She eventually finds pr2 and then needs to address it with Bob, then maybe help Bob merge pr1 so pr3 can use the latest code in main to avoid reverting pr2

This is very complicated and can be addressed by simply changing the default targeted branch to plan and apply to be master/main.

- id: /.*/
  # branch is an regex matching pull requests by base branch
  # (the branch the pull request is getting merged into).
  # By default, all branches are matched
  # branch: /.*/
  branch: /^(master|main)$/

Describe the solution you'd like

Instead of the default target branch of .*, it would be ideal for it to be the default branch of the repo if the branch key is omitted.

Describe the drawbacks of your solution

Documentation

Describe alternatives you've considered

Maybe change the default to be /^(master|main)$/