Favor strings.Cut to strings.SplitN

It is more efficient since it does not allocate an array and just return
s if nothing found

For the same reason we don't need to use  strings.Cut at all if empty

mapstructure.go

@@ -1358,8 +1358,8 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 		fieldName := field.Name
 
 		tagValue := field.Tag.Get(d.config.TagName)
-		tagValue = strings.SplitN(tagValue, ",", 2)[0]
 		if tagValue != "" {
+			tagValue, _, _ = strings.Cut(tagValue, ",")
 			fieldName = tagValue
 		}