Fix scan execution policy creation issue

- when a new project is created, the default branch was not
  being retrieved with it
- retrieve default branch
- update tests

ee/app/assets/javascripts/threat_monitoring/components/policy_editor/scan_execution_policy/lib/utils.js

@@ -31,12 +31,12 @@ const assignSecurityPolicyProject = async (projectPath) => {
     },
   });
 
-  return { ...project, errors };
+  return { ...project, branch: project?.branch?.rootRef, errors };
 };
 
 /**
  * Creates a merge request for the changes to the policy file
- * @param {Object} payload contains the path to the project, the branch to merge on the project, and the branch to merge into
+ * @param {Object} payload contains the path to the parent project, the branch to merge on the project, and the branch to merge into
  * @returns {Object} contains the id of the merge request and any errors
  */
 const createMergeRequest = async ({ projectPath, sourceBranch, targetBranch }) => {
@@ -98,7 +98,7 @@ export const savePolicy = async ({ assignedPolicyProject, projectPath, yamlEdito
   checkForErrors(currentAssignedPolicyProject);
 
   const newPolicyCommitBranch = await updatePolicy({
-    projectPath: currentAssignedPolicyProject.fullPath,
+    projectPath,
     yamlEditorValue,
   });
 

ee/app/assets/javascripts/threat_monitoring/graphql/mutations/create_policy_project.mutation.graphql

@@ -3,6 +3,9 @@ mutation createPolicyProject($projectPath: ID!) {
     project {
       fullPath
       id
+      branch: repository {
+        rootRef
+      }
     }
     errors
   }

ee/spec/frontend/threat_monitoring/components/policy_editor/scan_execution_policy/lib/utils_spec.js

@@ -8,7 +8,11 @@ import createMergeRequestMutation from '~/graphql_shared/mutations/create_merge_
 jest.mock('ee/threat_monitoring/utils');
 
 const defaultAssignedPolicyProject = { fullPath: 'path/to/policy-project', branch: 'main' };
-const newAssignedPolicyProject = { fullPath: 'path/to/new-project', branch: 'main' };
+const newAssignedPolicyProject = {
+  id: '02',
+  fullPath: 'path/to/new-project',
+  branch: { rootRef: 'main' },
+};
 const projectPath = 'path/to/current-project';
 const yamlEditorValue = 'some yaml';
 const createSavePolicyInput = (assignedPolicyProject = defaultAssignedPolicyProject) => ({
@@ -56,7 +60,12 @@ describe('savePolicy', () => {
       createSavePolicyInput(DEFAULT_ASSIGNED_POLICY_PROJECT),
     );
 
-    expect(currentAssignedPolicyProject).toStrictEqual({ ...newAssignedPolicyProject, errors: [] });
+    expect(currentAssignedPolicyProject).toStrictEqual({
+      id: newAssignedPolicyProject.id,
+      fullPath: newAssignedPolicyProject.fullPath,
+      branch: newAssignedPolicyProject.branch.rootRef,
+      errors: [],
+    });
     expect(mergeRequest).toStrictEqual({ id: '01', errors: [] });
   });