Disallow branch filters for tag push event

We cannot filter branches for a tag push event, since the
tag is referring to a certain commit and has no reference to
a branch. Therefore the webhook does not contain the information
about the branch.

Fixes #13206

src/api/app/validators/workflow_filters_validator.rb

@@ -27,6 +27,8 @@ def validate_filters
                            "#{unsupported_filters.keys.to_sentence} are unsupported")
     end
 
+    @workflow.errors.add(:filters, 'for branches are not supported for the tag push event') if unsupported_filters_for_event?
+
     return if unsupported_filter_values.blank?
 
     @workflow.errors.add(:filters, "#{unsupported_filter_values.to_sentence} have unsupported values, " \
@@ -56,4 +58,9 @@ def unsupported_filter_values
       unsupported_filter_values
     end
   end
+
+  def unsupported_filters_for_event?
+    # Tags do not have a reference to a branch, they are referring to a commit
+    @workflow_instructions[:filters][:branches].present? && @workflow.scm_webhook.tag_push_event?
+  end
 end