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

modules/convert/pull.go

@@ -68,6 +68,7 @@ func ToAPIPullRequest(ctx context.Context, pr *models.PullRequest, doer *user_mo
 		PatchURL:  pr.Issue.PatchURL(),
 		HasMerged: pr.HasMerged,
 		MergeBase: pr.MergeBase,
+		Mergeable: pr.Mergeable(),
 		Deadline:  apiIssue.Deadline,
 		Created:   pr.Issue.CreatedUnix.AsTimePtr(),
 		Updated:   pr.Issue.UpdatedUnix.AsTimePtr(),
@@ -191,10 +192,6 @@ func ToAPIPullRequest(ctx context.Context, pr *models.PullRequest, doer *user_mo
 		}
 	}
 
-	if pr.Status != models.PullRequestStatusChecking {
-		mergeable := !(pr.Status == models.PullRequestStatusConflict || pr.Status == models.PullRequestStatusError) && !pr.IsWorkInProgress()
-		apiPullRequest.Mergeable = mergeable
-	}
 	if pr.HasMerged {
 		apiPullRequest.Merged = pr.MergedUnix.AsTimePtr()
 		apiPullRequest.MergedCommitID = &pr.MergedCommitID