Allow --tag and --keep-tag to match untagged snapshots

internal/restic/snapshot.go

@@ -195,6 +195,9 @@ func (sn *Snapshot) hasTag(tag string) bool {
 // HasTags returns true if the snapshot has all the tags in l.
 func (sn *Snapshot) HasTags(l []string) bool {
 	for _, tag := range l {
+		if tag == "" && len(sn.Tags) == 0 {
+			return true
+		}
 		if !sn.hasTag(tag) {
 			return false
 		}

internal/restic/snapshot_test.go

@@ -14,3 +14,13 @@ func TestNewSnapshot(t *testing.T) {
 	_, err := restic.NewSnapshot(paths, nil, "foo", time.Now())
 	rtest.OK(t, err)
 }
+
+func TestTagList(t *testing.T) {
+	paths := []string{"/home/foobar"}
+	tags := []string{""}
+
+	sn, _ := restic.NewSnapshot(paths, nil, "foo", time.Now())
+
+	r := sn.HasTags(tags)
+	rtest.Assert(t, r, "Failed to match untagged snapshot")
+}