(release/v1.2) fix(Dgraph): Sort manifests by backup number. (#6281)

Original change was not cherry-picked originally because the change was
made during the addition of online restores, which is a new feature in
the 20.07 version.

ee/backup/file_handler.go

@@ -145,6 +145,12 @@ func (h *fileHandler) Load(uri *url.URL, backupId string, fn loadFn) LoadResult
 		return LoadResult{0, 0, err}
 	}
 
+	// Sort manifests in the ascending order of their BackupNum so that the first
+	// manifest corresponds to the first full backup and so on.
+	sort.Slice(manifests, func(i, j int) bool {
+		return manifests[i].BackupNum < manifests[j].BackupNum
+	})
+
 	// Process each manifest, first check that they are valid and then confirm the
 	// backup files for each group exist. Each group in manifest must have a backup file,
 	// otherwise this is a failure and the user must remedy.

ee/backup/s3_handler.go

@@ -283,6 +283,12 @@ func (h *s3Handler) Load(uri *url.URL, backupId string, fn loadFn) LoadResult {
 		return LoadResult{0, 0, err}
 	}
 
+	// Sort manifests in the ascending order of their BackupNum so that the first
+	// manifest corresponds to the first full backup and so on.
+	sort.Slice(manifests, func(i, j int) bool {
+		return manifests[i].BackupNum < manifests[j].BackupNum
+	})
+
 	// Process each manifest, first check that they are valid and then confirm the
 	// backup manifests for each group exist. Each group in manifest must have a backup file,
 	// otherwise this is a failure and the user must remedy.