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

services/pull/patch.go

@@ -444,14 +444,16 @@ func checkConflicts(ctx context.Context, pr *models.PullRequest, gitRepo *git.Re
 			},
 		})
 
-	// 9. If there is a conflict the `git apply` command will return a non-zero error code - so there will be a positive error.
-	if err != nil {
+	// 9. Check if the found conflictedfiles is non-zero, "err" could be non-nil, so we should ignore it if we found conflicts.
+	// Note: `"err" could be non-nil` is due that if enable 3-way merge, it doesn't return any error on found conflicts.
+	if len(pr.ConflictedFiles) > 0 {
 		if conflict {
 			pr.Status = models.PullRequestStatusConflict
 			log.Trace("Found %d files conflicted: %v", len(pr.ConflictedFiles), pr.ConflictedFiles)
 
 			return true, nil
 		}
+	} else if err != nil {
 		return false, fmt.Errorf("git apply --check: %v", err)
 	}
 	return false, nil