Actually use retval

This is most likely a bug. Nocited by gcc 4.6.0.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

tree-diff.c

@@ -58,7 +58,7 @@ static int compare_tree_entry(struct tree_desc *t1, struct tree_desc *t2,
 		opt->change(opt, mode1, mode2, sha1, sha2, base->buf, 0, 0);
 	}
 	strbuf_setlen(base, old_baselen);
-	return 0;
+	return retval;
 }
 
 /* A whole sub-tree went away or appeared */