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

* Set correct PR status on 3way on conflict checking

- 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.

* Fix linter

1 files changed, 11 insertions, 0 deletions

diff --git a/models/pull.go b/models/pull.go
index 439005deb4..ac44ebf0be 100644
--- a/models/pull.go
+++ b/models/pull.go
@@ -701,3 +701,14 @@ func (pr *PullRequest) GetHeadBranchHTMLURL() string {
 	}
 	return pr.HeadRepo.HTMLURL() + "/src/branch/" + util.PathEscapeSegments(pr.HeadBranch)
 }
+
+// Mergeable returns if the pullrequest is mergeable.
+func (pr *PullRequest) Mergeable() bool {
+	// If a pull request isn't mergable if it's:
+	// - Being conflict checked.
+	// - Has a conflict.
+	// - Received a error while being conflict checked.
+	// - Is a work-in-progress pull request.
+	return pr.Status != PullRequestStatusChecking && pr.Status != PullRequestStatusConflict &&
+		pr.Status != PullRequestStatusError && !pr.IsWorkInProgress()
+}