Fix epic dates not updating with subgroups

The change in https://gitlab.com/gitlab-org/gitlab/merge_requests/22467
caused the wrong path to be passed to the GraphQL endpoint. The frontend
passed only the immediate path (e.g. charts) instead of the full path
(e.g. gitlab-org/charts).

Closes https://gitlab.com/gitlab-org/gitlab/issues/197495

ee/app/assets/javascripts/epic/store/actions.js

@@ -130,7 +130,7 @@ export const requestEpicDateSaveFailure = ({ commit }, data) => {
 export const saveDate = ({ state, dispatch }, { dateType, dateTypeIsFixed, newDate }) => {
   const updateEpicInput = {
     iid: `${state.epicIid}`,
-    groupPath: state.groupPath,
+    groupPath: state.fullPath,
     [dateType === dateTypes.start ? 'startDateIsFixed' : 'dueDateIsFixed']: dateTypeIsFixed,
   };