tag: added function tag_merge_replace()

Like tag_merge(), but can deal with NULL parameters, and frees both
tag objects.

src/tag.h

@@ -165,6 +165,15 @@ struct tag *tag_dup(const struct tag *tag);
 struct tag *
 tag_merge(const struct tag *base, const struct tag *add);
 
+/**
+ * Merges the data from two tags.  Any of the two may be NULL.  Both
+ * are freed by this function.
+ *
+ * @return a newly allocated tag, which must be freed with tag_free()
+ */
+struct tag *
+tag_merge_replace(struct tag *base, struct tag *add);
+
 /**
  * Returns true if the tag contains no items.  This ignores the "time"
  * attribute.