g.mlist: suppress messages for non-terminal output

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@60820 15284696-431f-4ddb-bdfa-cd5b030d7da7

general/g.mlist/main.c

@@ -315,7 +315,7 @@ static void make_list(FILE *fp, const struct list *elem, const char *mapset,
     if (count > 0) {
 	if (any)
 	    fprintf(fp, "%s", separator);
-	if (fp == stdout)
+	if (fp == stdout && isatty(STDOUT_FILENO))
 	    G_message(_("%s available in mapset <%s>:"), elem->text, mapset);
     }