when using distinct cols with FindAndCount, reset statement.ColumnMap will make the counting sql an syntax error, this pr try fix this.
is this pr ok for merge?
error sql logging:
```sql
[SQL] SELECT DISTINCT `Fid` FROM `table_demo` WHERE Fkey = ? [val]
[SQL] SELECT count(DISTINCT ) FROM `table_demo` WHERE Fkey = ? [val]
```
after fix:
```sql
[SQL] SELECT DISTINCT `Fid` FROM `table_demo` WHERE Fkey = ? [val]
[SQL] SELECT count(DISTINCT `Fid`) FROM `table_demo` WHERE Fkey = ? [val]
```
Co-authored-by: finelog <kaicltw@gmail.com>
Reviewed-on: xorm/xorm#2096
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: finelog <finelog@noreply.gitea.io>
Co-committed-by: finelog <finelog@noreply.gitea.io>