grep: fix leak in grep_splice_or()

In `grep_splice_or()` we search for the next `TRUE` node in our tree of
grep expressions and replace it with the given new expression. But we
don't free the old node, which causes a memory leak. Plug it.

This leak is exposed by t7810, but plugging it alone isn't sufficient to
make the test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -756,6 +756,7 @@ static struct grep_expr *grep_splice_or(struct grep_expr *x, struct grep_expr *y
 		assert(x->node == GREP_NODE_OR);
 		if (x->u.binary.right &&
 		    x->u.binary.right->node == GREP_NODE_TRUE) {
+			free(x->u.binary.right);
 			x->u.binary.right = y;
 			break;
 		}