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/ex_cmds.c b/src/ex_cmds.c
index 6e713c4..0a87f7b 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -2117,7 +2117,7 @@
 		if (st_old.st_uid != tmp_st.st_uid)
 		    /* Changing the owner might fail, in which case the
 		     * file will now owned by the current user, oh well. */
-		    ignored = fchown(fileno(fp_out), st_old.st_uid, -1);
+		    vim_ignored = fchown(fileno(fp_out), st_old.st_uid, -1);
 		if (st_old.st_gid != tmp_st.st_gid
 			&& fchown(fileno(fp_out), -1, st_old.st_gid) == -1)
 		    /* can't set the group to what it should be, remove