rebuild-index: Ignore broken index files

Previously, an e.g. truncated index could prevent rebuild-index from
working.

cmd/restic/cmd_rebuild_index.go

@@ -73,7 +73,27 @@ func rebuildIndex(opts RebuildIndexOptions, gopts GlobalOptions, repo *repositor
 		}
 	} else {
 		Verbosef("loading indexes...\n")
-		err := repo.LoadIndex(gopts.ctx)
+		mi := repository.NewMasterIndex()
+		err := repository.ForAllIndexes(ctx, repo, func(id restic.ID, idx *repository.Index, oldFormat bool, err error) error {
+			if err != nil {
+				Warnf("removing invalid index %v: %v\n", id, err)
+				obsoleteIndexes = append(obsoleteIndexes, id)
+				return nil
+			}
+
+			mi.Insert(idx)
+			return nil
+		})
+		if err != nil {
+			return err
+		}
+
+		err = mi.MergeFinalIndexes()
+		if err != nil {
+			return err
+		}
+
+		err = repo.SetIndex(mi)
 		if err != nil {
 			return err
 		}