patch 8.1.0377: xdiff doesn't use the Vim memory allocation functions

Problem:    Xdiff doesn't use the Vim memory allocation functions.
Solution:   Change the xdl_ defines.  Check for out-of-memory.  Rename
            "ignored" to "vim_ignored".

src/main.c

@@ -2632,7 +2632,7 @@ read_stdin(void)
      * Is there any other system that cannot do this?
      */
     close(0);
-    ignored = dup(2);
+    vim_ignored = dup(2);
 #endif
 }