In Gitlab, if an Atlantis 'Apply' Pipeline Job fails, it cannot be Re-Applied on the Same Commit

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Overview of the Issue

If an Atlantis 'apply' pipeline job fails, using Gitlab with 'Pipelines must succeed' configured on the repo and Atlantis apply_requirements: mergeable set, Atlantis will reject any attempt to re-run the apply on the same commit with the error 'Pull request must be mergeable before running apply.'

Environment details

• Atlantis version: 0.26.0

Atlantis server-side config file:

repos:
- id: /.*/
  apply_requirements: mergeable

Additional Context

This is due to the GitLab client PullIsMergeable function not considering that a pull request is mergeable if the mr.DetailedMergeStatus has a value of ci_must_pass:

atlantis/server/events/vcs/gitlab_client.go

Line 331 in 6ff0e2f

if ((ok && (mr.DetailedMergeStatus == "mergeable" || mr.DetailedMergeStatus == "ci_still_running")) ||

This will occur if GitLab 'Pipelines must succeed' is configured on the repo and an Atlantis apply pipeline job has previously failed on the commit, therefore as far as GitLab is concerned, not all the CI jobs have succeeded, therefore the DetailedMergeStatus will be ci_must_pass.

The function already individually checks all the commit statuses, and will return false if any other status other than Atlantis Apply statuses do not have a status of success, so allowing this status will not stop other non-Atlantis failed CI checks from preventing an Atlantis apply.

GitLab Merge Status Reference: https://docs.gitlab.com/ee/api/merge_requests.html#merge-status