Disable merge operation when it would damage relations

The operation is disabled when attempting to join ways which don't belong to identical sets of relations. Restriction relations are excluded, because they are already handled with slightly different logic.

Fixes #8674
Fixes #8645
Fixes #3825
Fixes #1512

modules/operations/merge.js

@@ -74,6 +74,9 @@ export function operationMerge(context, selectedIDs) {
     operation.tooltip = function() {
         var disabled = operation.disabled();
         if (disabled) {
+            if (disabled === 'conflicting_relations') {
+                return t('operations.merge.conflicting_relations');
+            }
             if (disabled === 'restriction') {
                 return t('operations.merge.restriction',
                     { relation: presetManager.item('type/restriction').name() });