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".
diff --git a/src/diff.c b/src/diff.c
index 59daf1e..0bf0ef2 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1203,10 +1203,10 @@
     {
 # ifdef TEMPDIRNAMES
 	if (vim_tempdir != NULL)
-	    ignored = mch_chdir((char *)vim_tempdir);
+	    vim_ignored = mch_chdir((char *)vim_tempdir);
 	else
 # endif
-	    ignored = mch_chdir("/tmp");
+	    vim_ignored = mch_chdir("/tmp");
 	shorten_fnames(TRUE);
     }
 #endif