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

tagcmp.go

@@ -174,14 +174,10 @@ func (curTag Tag) UpgradeElement(newTag Tag) (Tag, error) {
 	}
 	diff := curTag
 	diff.Major(Itoa(Atoi(newTag.Major) - Atoi(curTag.Major)))
-	if curTag.MissingMinor {
-		diff.Minor = nil
-	} else {
+	if !curTag.MissingMinor {
 		diff.Minor(Itoa(Atoi(newTag.Minor) - Atoi(curTag.Minor)))
 	}
-	if curTag.MissingPatch {
-		diff.Patch = nil
-	} else {
+	if !curTag.MissingPatch {
 		diff.Patch(Itoa(Atoi(newTag.Patch) - Atoi(curTag.Patch)))
 	}