sql/postgres: add missing comma in column changes for reverse changes in Postgres

[tprebs]

Adds the missing comma when generating reverse changes on columns. #667

This change just adds the comma the same way it has been done for other changes and in the mysql migration but it doesn't feel right to have to manually add the comma in when generating the reverse sql when it is done automatically by MapComma for alter table cmd.

Would it make more sense to either split the logic for reverse out into a second iteration via reversible.MapComma create a wrapper struct which exposes both builders to the existing code e.g. rb.MapComma(changes, func(i int, b *sqlx.Builder, rb *sqlx.Builder)) This would mean in the future the logic for applying commas is standard between the generation of the cmd and reverse

[a8m]

Thanks for the contribution @tprebs.

Regarding your suggestion, I agree it can be improved. I'll try to think of a solution for this.