Fix URL parsing in IDE MRs

**Why?**
A recent MR moved the `merge_requests` route under
the `/-/` namespace. This broke our URL parsing for the
project path.

https://gitlab.com/gitlab-org/gitlab/issues/200087#note_280354290

app/assets/javascripts/ide/stores/modules/merge_requests/mutations.js

@@ -14,9 +14,10 @@ export default {
       iid: mergeRequest.iid,
       title: mergeRequest.title,
       projectId: mergeRequest.project_id,
-      projectPathWithNamespace: mergeRequest.web_url
-        .replace(`${gon.gitlab_url}/`, '')
-        .replace(`/merge_requests/${mergeRequest.iid}`, ''),
+      projectPathWithNamespace: mergeRequest.references.full.replace(
+        mergeRequest.references.short,
+        '',
+      ),
     }));
   },
   [types.RESET_MERGE_REQUESTS](state) {

spec/frontend/ide/mock_data.js

@@ -165,7 +165,11 @@ export const mergeRequests = [
     iid: 1,
     title: 'Test merge request',
     project_id: 1,
-    web_url: `${TEST_HOST}/namespace/project-path/merge_requests/1`,
+    web_url: `${TEST_HOST}/namespace/project-path/-/merge_requests/1`,
+    references: {
+      short: '!1',
+      full: 'namespace/project-path!1',
+    },
   },
 ];