Use diff metadata to find out when to send diff-files-end perf mark

We were using `state.diffs.length` to find out whether the index
of diff item is the last of all diffs. But the `state.diffs.length`
doesn't reflect the total, but only the number of currently
fetched items. This change uses `diffFilesLength` instead, which is
taken from the diff metadata and reflects the total number of changed
files.

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

@@ -522,7 +522,7 @@ export default {
               :file="file"
               :reviewed="fileReviews[index]"
               :is-first-file="index === 0"
-              :is-last-file="index === diffs.length - 1"
+              :is-last-file="index === diffFilesLength - 1"
               :help-page-path="helpPagePath"
               :can-current-user-fork="canCurrentUserFork"
               :view-diffs-file-by-file="viewDiffsFileByFile"