forget: refuse deleting the last snapshot in a snapshot group

`--keep-tag invalid-tag` was previously able to wipe all snapshots in a
repository. As a user specified a `--keep-*` option this is likely
unintentional. This forbid deleting all snapshot if a `--keep-*` option
was specified to prevent data loss. (Not specifying such an option
currently also causes the command to abort)

internal/restic/snapshot_find.go

@@ -24,7 +24,7 @@ type SnapshotFilter struct {
 	TimestampLimit time.Time
 }
 
-func (f *SnapshotFilter) empty() bool {
+func (f *SnapshotFilter) Empty() bool {
 	return len(f.Hosts)+len(f.Tags)+len(f.Paths) == 0
 }
 
@@ -173,7 +173,7 @@ func (f *SnapshotFilter) FindAll(ctx context.Context, be Lister, loader LoaderUn
 		}
 
 		// Give the user some indication their filters are not used.
-		if !usedFilter && !f.empty() {
+		if !usedFilter && !f.Empty() {
 			return fn("filters", nil, errors.Errorf("explicit snapshot ids are given"))
 		}
 		return nil

internal/restic/snapshot_group.go

@@ -66,6 +66,20 @@ type SnapshotGroupKey struct {
 	Tags     []string `json:"tags"`
 }
 
+func (s *SnapshotGroupKey) String() string {
+	var parts []string
+	if s.Hostname != "" {
+		parts = append(parts, fmt.Sprintf("host %v", s.Hostname))
+	}
+	if len(s.Paths) != 0 {
+		parts = append(parts, fmt.Sprintf("path %v", s.Paths))
+	}
+	if len(s.Tags) != 0 {
+		parts = append(parts, fmt.Sprintf("tags %v", s.Tags))
+	}
+	return strings.Join(parts, ", ")
+}
+
 // GroupSnapshots takes a list of snapshots and a grouping criteria and creates
 // a grouped list of snapshots.
 func GroupSnapshots(snapshots Snapshots, groupBy SnapshotGroupByOptions) (map[string]Snapshots, bool, error) {