Snapshot: Add `AddTags()` and `RemoveTags()`

Both prevent duplicate tags.

src/cmds/restic/cmd_tag.go

@@ -62,38 +62,16 @@ func changeTags(repo *repository.Repository, snapshotID restic.ID, setTags, addT
 	}
 
 	if len(setTags) != 0 {
-		// Setting the tag to an empty string really means no more tags.
+		// Setting the tag to an empty string really means no tags.
 		if len(setTags) == 1 && setTags[0] == "" {
 			setTags = nil
 		}
 		sn.Tags = setTags
 		changed = true
 	} else {
-		for _, add := range addTags {
-			found := false
-			for _, tag := range sn.Tags {
-				if tag == add {
-					found = true
-					break
-				}
-			}
-			if !found {
-				sn.Tags = append(sn.Tags, add)
-				changed = true
-			}
-		}
-		for _, remove := range removeTags {
-			for i, tag := range sn.Tags {
-				if tag == remove {
-					// https://github.com/golang/go/wiki/SliceTricks
-					sn.Tags[i] = sn.Tags[len(sn.Tags)-1]
-					sn.Tags[len(sn.Tags)-1] = ""
-					sn.Tags = sn.Tags[:len(sn.Tags)-1]
-
-					changed = true
-					break
-				}
-			}
+		changed = sn.AddTags(addTags)
+		if sn.RemoveTags(removeTags) {
+			changed = true
 		}
 	}
 

src/restic/snapshot.go

@@ -73,8 +73,7 @@ func LoadAllSnapshots(repo Repository) (snapshots []*Snapshot, err error) {
 
 		snapshots = append(snapshots, sn)
 	}
-
-	return snapshots, nil
+	return
 }
 
 func (sn Snapshot) String() string {
@@ -99,6 +98,41 @@ func (sn *Snapshot) fillUserInfo() error {
 	return err
 }
 
+// AddTags adds the given tags to the snapshots tags, preventing duplicates.
+// It returns true if any changes were made.
+func (sn *Snapshot) AddTags(addTags []string) (changed bool) {
+nextTag:
+	for _, add := range addTags {
+		for _, tag := range sn.Tags {
+			if tag == add {
+				continue nextTag
+			}
+		}
+		sn.Tags = append(sn.Tags, add)
+		changed = true
+	}
+	return
+}
+
+// RemoveTags removes the given tags from the snapshots tags and
+// returns true if any changes were made.
+func (sn *Snapshot) RemoveTags(removeTags []string) (changed bool) {
+	for _, remove := range removeTags {
+		for i, tag := range sn.Tags {
+			if tag == remove {
+				// https://github.com/golang/go/wiki/SliceTricks
+				sn.Tags[i] = sn.Tags[len(sn.Tags)-1]
+				sn.Tags[len(sn.Tags)-1] = ""
+				sn.Tags = sn.Tags[:len(sn.Tags)-1]
+
+				changed = true
+				break
+			}
+		}
+	}
+	return
+}
+
 // HasTags returns true if the snapshot has all the tags.
 func (sn *Snapshot) HasTags(tags []string) bool {
 nextTag: