Use actual file IDs to retrieve reviews

It's not very robust to retrieve a value by its position in an array.

Instead, because the files all have unique IDs we return an
object where each key is a file ID and it's value is whether that
file has been marked as reviewed.

This fixes the issue where the list of files in single-file mode
always has a length of 1, which means the fetched review is
always the first file in the MR.

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

@@ -514,7 +514,7 @@ export default {
               v-for="(file, index) in diffs"
               :key="file.newPath"
               :file="file"
-              :reviewed="fileReviews[index]"
+              :reviewed="fileReviews[file.id]"
               :is-first-file="index === 0"
               :is-last-file="index === diffFilesLength - 1"
               :help-page-path="helpPagePath"

app/assets/javascripts/diffs/utils/file_reviews.js

@@ -9,7 +9,12 @@ export function isFileReviewed(reviews, file) {
 }
 
 export function reviewStatuses(files, reviews) {
-  return files.map((file) => isFileReviewed(reviews, file));
+  return files.reduce((flat, file) => {
+    return {
+      ...flat,
+      [file.id]: isFileReviewed(reviews, file),
+    };
+  }, {});
 }
 
 export function getReviewsForMergeRequest(mrPath) {

spec/frontend/diffs/utils/file_reviews_spec.js

@@ -49,11 +49,11 @@ describe('File Review(s) utilities', () => {
 
     it.each`
       mrReviews                         | files             | fileReviews
-      ${{}}                             | ${[file1, file2]} | ${[false, false]}
-      ${{ abc: ['123'] }}               | ${[file1, file2]} | ${[true, false]}
-      ${{ abc: ['098'] }}               | ${[file1, file2]} | ${[false, true]}
-      ${{ def: ['123'] }}               | ${[file1, file2]} | ${[false, false]}
-      ${{ abc: ['123'], def: ['098'] }} | ${[]}             | ${[]}
+      ${{}}                             | ${[file1, file2]} | ${{ 123: false, '098': false }}
+      ${{ abc: ['123'] }}               | ${[file1, file2]} | ${{ 123: true, '098': false }}
+      ${{ abc: ['098'] }}               | ${[file1, file2]} | ${{ 123: false, '098': true }}
+      ${{ def: ['123'] }}               | ${[file1, file2]} | ${{ 123: false, '098': false }}
+      ${{ abc: ['123'], def: ['098'] }} | ${[]}             | ${{}}
     `(
       'returns $fileReviews based on the diff files in state and the existing reviews $reviews',
       ({ mrReviews, files, fileReviews }) => {