Use array instead of hash for backend.ID

Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).

This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.

cmd/restic/cmd_ls.go

@@ -56,7 +56,7 @@ func (cmd CmdLs) printTree(prefix string, repo *repository.Repository, id backen
 		cmd.global.Printf(cmd.printNode(prefix, entry) + "\n")
 
 		if entry.Type == "dir" && entry.Subtree != nil {
-			err = cmd.printTree(filepath.Join(prefix, entry.Name), repo, entry.Subtree)
+			err = cmd.printTree(filepath.Join(prefix, entry.Name), repo, *entry.Subtree)
 			if err != nil {
 				return err
 			}
@@ -97,5 +97,5 @@ func (cmd CmdLs) Execute(args []string) error {
 
 	cmd.global.Verbosef("snapshot of %v at %s:\n", sn.Paths, sn.Time)
 
-	return cmd.printTree("", repo, sn.Tree)
+	return cmd.printTree("", repo, *sn.Tree)
 }