Set correct PR status on 3way on conflict checking (#19457) (#19458)

Set correct PR status on 3way on conflict checking (#19457) (#19458)

- Backport #19457
  - When 3-way merge is enabled for conflict checking, it has a new interesting behavior that it doesn't return any error when it found a conflict, so we change the condition to not check for the error, but instead check if conflictedfiles is populated, this fixes a issue whereby PR status wasn't correctly on conflicted PR's.
  - Refactor the mergeable property(which was incorrectly set and lead me this bug) to be more maintainable.
  - Add a dedicated test for conflicting checking, so it should prevent future issues with this.
  - Ref: Fix the latest error for https://gitea.com/gitea/go-sdk/pulls/579

Co-authored-by: zeripath <art27@cantab.net>