feat: add Tag.UpgradeElement(newTag Tag) which returns a difference between two tags

tagcmp.go

@@ -166,6 +166,24 @@ func (t Tag) IsCompatible(tag Tag) bool {
 	return true
 }
 
+// UpgradeElement returns a Tag object which is the difference between an old and new tag
+// It can contain negative numbers if comparing with an older tag.
+func (curTag Tag) UpgradeElement(newTag Tag) (Tag, error) {
+	if !curTag.IsCompatible(newTag) {
+		return Tag{}, fmt.Errorf("%s and %s are not compatible with each other", curTag.String(), newTag.String())
+	}
+	diff := curTag
+	diff.Major(Itoa(Atoi(newTag.Major) - Atoi(curTag.Major)))
+	if !curTag.MissingMinor {
+		diff.Minor(Itoa(Atoi(newTag.Minor) - Atoi(curTag.Minor)))
+	}
+	if !curTag.MissingPatch {
+		diff.Patch(Itoa(Atoi(newTag.Patch) - Atoi(curTag.Patch)))
+	}
+
+	return diff, nil
+}
+
 // CommitHashPattern matches commit-like hash tags
 var CommitHashPattern = "^[a-f0-9]{7,40}$"