Fails to rename column when alter renames + re-adds new column with prev name

When renaming a column and then re-creating it within the same ALTER statement, gh-ost deletes the entiretity of the name-changed column.

Example:

```sql
-- regular alter table (expected behaviour)
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY auto_increment, c1 VARCHAR(100), c2 VARCHAR(100));
INSERT INTO t1 (c1, c2) VALUES ('a', 'b'), ('c', 'd'), ('e', 'f');
ALTER TABLE t1 RENAME COLUMN c1 TO n1, ADD COLUMN c1 varchar(100);
SELECT * FROM t1;
+----+------+------+------+
| id | n1 | c2 | c1 |
+----+------+------+------+
| 1 | a | b | NULL |
| 2 | c | d | NULL |
| 3 | e | f | NULL |
+----+------+------+------+
```

Using gh-ost:

```bash
go run . \
--max-load=Threads_running=25 \
--critical-load=Threads_running=1000 \
--chunk-size=1000 \
--user="[REDACTED]" \
--password="[REDACTED]" \
--host="127.0.0.1" \
--port=8030 \
--allow-on-master \
--database="test" \
--table="t1" \
--verbose \
--initially-drop-old-table \
--ok-to-drop-table \
--execute --initially-drop-ghost-table --alter="RENAME COLUMN c1 TO n1, ADD COLUMN c1 varchar(100)"
```

Result:
```sql
+----+------+------+------+
| id | n1 | c2 | c1 |
+----+------+------+------+
| 1 | NULL | b | a |
| 2 | NULL | d | c |
| 3 | NULL | f | e |
+----+------+------+------+
```