Move translation into the existing i18n file

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

@@ -88,7 +88,7 @@ export default {
           :checked="showWhitespace"
           @change="setShowWhitespace({ showWhitespace: $event.target.checked, pushState: true })"
         />
-        {{ __('Show whitespace changes') }}
+        {{ $options.i18n.whitespace }}
       </label>
     </div>
     <div class="gl-mt-3 gl-px-3">

app/assets/javascripts/diffs/i18n.js

@@ -21,5 +21,6 @@ export const DIFF_FILE = {
 };
 
 export const SETTINGS_DROPDOWN = {
+  whitespace: __('Show whitespace changes'),
   fileByFile: __('Show one file at a time'),
 };