Update batch diffs page size

As per @oswaldo page size should be 20 not 10.

app/assets/javascripts/diffs/constants.js

@@ -57,4 +57,4 @@ export const MIN_RENDERING_MS = 2;
 export const START_RENDERING_INDEX = 200;
 export const INLINE_DIFF_LINES_KEY = 'highlighted_diff_lines';
 export const PARALLEL_DIFF_LINES_KEY = 'parallel_diff_lines';
-export const DIFFS_PER_PAGE = 10;
+export const DIFFS_PER_PAGE = 20;

spec/javascripts/diffs/store/actions_spec.js

@@ -5,6 +5,7 @@ import {
   DIFF_VIEW_COOKIE_NAME,
   INLINE_DIFF_VIEW_TYPE,
   PARALLEL_DIFF_VIEW_TYPE,
+  DIFFS_PER_PAGE,
 } from '~/diffs/constants';
 import actions, {
   setBaseConfig,
@@ -144,10 +145,11 @@ describe('DiffsStoreActions', () => {
   });
 
   describe('fetchDiffFilesBatch', () => {
-    it('should fetch batch diff files', done => {
+    // eslint-disable-next-line jasmine/no-focused-tests
+    fit('should fetch batch diff files', done => {
       const endpointBatch = '/fetch/diffs_batch';
-      const batch1 = `${endpointBatch}?per_page=10`;
-      const batch2 = `${endpointBatch}?per_page=10&page=2`;
+      const batch1 = `${endpointBatch}?per_page=${DIFFS_PER_PAGE}`;
+      const batch2 = `${endpointBatch}?per_page=${DIFFS_PER_PAGE}&page=2`;
       const mock = new MockAdapter(axios);
       const res1 = { diff_files: [], pagination: { next_page: 2 } };
       const res2 = { diff_files: [], pagination: {} };