Transforms diffs module to a namespaced one to avoid console error due to rewire export.

Detailed: Because of rewire we need to export a default empty object in our actions to prevent it to export the wrong default in karma. Vuex getters are global, and because the Vuex store uses several non namespaced moduled, there was already a getter named default, due to the same empty export.
In order to solve it I chose to namespace the module. Could also be fixed by importing the getters explicitly instead of all of them.

app/assets/javascripts/diffs/components/app.vue

@@ -63,7 +63,8 @@ export default {
       plainDiffPath: state => state.diffs.plainDiffPath,
       emailPatchPath: state => state.diffs.emailPatchPath,
     }),
-    ...mapGetters(['isParallelView', 'isNotesFetched']),
+    ...mapGetters('diffs', ['isParallelView']),
+    ...mapGetters(['isNotesFetched']),
     targetBranch() {
       return {
         branchName: this.targetBranchName,
@@ -115,7 +116,7 @@ export default {
     this.adjustView();
   },
   methods: {
-    ...mapActions(['setBaseConfig', 'fetchDiffFiles']),
+    ...mapActions('diffs', ['setBaseConfig', 'fetchDiffFiles']),
     fetchData() {
       this.fetchDiffFiles().catch(() => {
         createFlash(__('Something went wrong on our end. Please try again!'));

app/assets/javascripts/diffs/components/changed_files.vue

@@ -31,7 +31,7 @@ export default {
     };
   },
   computed: {
-    ...mapGetters(['isInlineView', 'isParallelView', 'areAllFilesCollapsed']),
+    ...mapGetters('diffs', ['isInlineView', 'isParallelView', 'areAllFilesCollapsed']),
     sumAddedLines() {
       return this.sumValues('addedLines');
     },
@@ -66,7 +66,7 @@ export default {
     document.removeEventListener('scroll', this.handleScroll);
   },
   methods: {
-    ...mapActions(['setInlineDiffViewType', 'setParallelDiffViewType', 'expandAllFiles']),
+    ...mapActions('diffs', ['setInlineDiffViewType', 'setParallelDiffViewType', 'expandAllFiles']),
     pluralize,
     handleScroll() {
       if (!this.updating) {

app/assets/javascripts/diffs/components/diff_content.vue

@@ -22,7 +22,7 @@ export default {
       projectPath: state => state.diffs.projectPath,
       endpoint: state => state.diffs.endpoint,
     }),
-    ...mapGetters(['isInlineView', 'isParallelView']),
+    ...mapGetters('diffs', ['isInlineView', 'isParallelView']),
     diffMode() {
       const diffModeKey = Object.keys(diffModes).find(key => this.diffFile[`${key}File`]);
       return diffModes[diffModeKey] || diffModes.replaced;

app/assets/javascripts/diffs/components/diff_file.vue

@@ -55,7 +55,7 @@ export default {
     document.removeEventListener('scroll', this.handleScroll);
   },
   methods: {
-    ...mapActions(['loadCollapsedDiff']),
+    ...mapActions('diffs', ['loadCollapsedDiff']),
     handleToggle() {
       const { collapsed, highlightedDiffLines, parallelDiffLines } = this.file;

app/assets/javascripts/diffs/components/diff_line_gutter_content.vue

@@ -108,7 +108,7 @@ export default {
     },
   },
   methods: {
-    ...mapActions(['loadMoreLines', 'showCommentForm']),
+    ...mapActions('diffs', ['loadMoreLines', 'showCommentForm']),
     handleCommentButton() {
       this.showCommentForm({ lineCode: this.lineCode });
     },

app/assets/javascripts/diffs/components/diff_line_note_form.vue

@@ -59,7 +59,8 @@ export default {
     }
   },
   methods: {
-    ...mapActions(['cancelCommentForm', 'saveNote', 'fetchDiscussions']),
+    ...mapActions('diffs', ['cancelCommentForm']),
+    ...mapActions(['saveNote', 'fetchDiscussions']),
     handleCancelCommentForm() {
       this.autosave.reset();
       this.cancelCommentForm({

app/assets/javascripts/diffs/components/inline_diff_table_row.vue

@@ -36,7 +36,7 @@ export default {
     };
   },
   computed: {
-    ...mapGetters(['isInlineView']),
+    ...mapGetters('diffs', ['isInlineView']),
     isContextLine() {
       return this.line.type === CONTEXT_LINE_TYPE;
     },

app/assets/javascripts/diffs/components/inline_diff_view.vue

@@ -20,7 +20,7 @@ export default {
     },
   },
   computed: {
-    ...mapGetters(['commitId']),
+    ...mapGetters('diffs', ['commitId']),
     normalizedDiffLines() {
       return this.diffLines.map(line => (line.richText ? trimFirstCharOfLineContent(line) : line));
     },

app/assets/javascripts/diffs/components/parallel_diff_table_row.vue

@@ -40,7 +40,7 @@ export default {
     };
   },
   computed: {
-    ...mapGetters(['isParallelView']),
+    ...mapGetters('diffs', ['isParallelView']),
     isContextLine() {
       return this.line.left.type === CONTEXT_LINE_TYPE;
     },

app/assets/javascripts/diffs/components/parallel_diff_view.vue

@@ -21,7 +21,7 @@ export default {
     },
   },
   computed: {
-    ...mapGetters(['commitId']),
+    ...mapGetters('diffs', ['commitId']),
     parallelDiffLines() {
       return this.diffLines.map(line => {
         const parallelLine = Object.assign({}, line);

app/assets/javascripts/diffs/store/modules/index.js

@@ -4,6 +4,7 @@ import mutations from '../mutations';
 import createState from './diff_state';
 export default {
+  namespaced: true,
   state: createState(),
   getters,
   actions,