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/window.c

@@ -4436,7 +4436,7 @@ win_enter_ext(
     {
 	/* Window doesn't have a local directory and we are not in the global
 	 * directory: Change to the global directory. */
-	ignored = mch_chdir((char *)globaldir);
+	vim_ignored = mch_chdir((char *)globaldir);
 	VIM_CLEAR(globaldir);
 	shorten_fnames(TRUE);
     }