Don't allow merging PR's which are being conflict checked (#19357) (#19358)

* Don't allow merging PR's which are being conflict checked (#19357)

- Backport of #19357
  - When a PR is still being conflict checked, don't allow the PR to be merged(the merge button could already be visible before e.g. a new commit was pushed to the PR).
  - Resolves #19352

* Update error message

services/pull/check.go

@@ -36,6 +36,7 @@ var (
 	ErrUserNotAllowedToMerge = errors.New("user not allowed to merge")
 	ErrHasMerged             = errors.New("has already been merged")
 	ErrIsWorkInProgress      = errors.New("work in progress PRs cannot be merged")
+	ErrIsChecking            = errors.New("cannot merge while conflict checking is in progress")
 	ErrNotMergableState      = errors.New("not in mergeable state")
 	ErrDependenciesLeft      = errors.New("is blocked by an open dependency")
 )
@@ -88,6 +89,10 @@ func CheckPullMergable(ctx context.Context, doer *user_model.User, perm *models.
 		return ErrNotMergableState
 	}
 
+	if pr.IsChecking() {
+		return ErrIsChecking
+	}
+
 	if err := CheckPRReadyToMerge(pr, false); err != nil {
 		if models.IsErrNotAllowedToMerge(err) {
 			if force {