`dolt_diff` assumes schema has not changed

`NewDiffTable` uses the root table schema as both the before and after schema when creating the `dolt_diff` schema ([code](https://github.com/dolthub/dolt/blob/6e9d403257b39a3e6e29bd5200c732e2d4611bfe/go/libraries/doltcore/sqle/dtables/diff_table.go#L106))

This will yield an inaccurate diff if columns have been added or deleted.

Start in go/libraries/doltcore/sqle/dtables/diff_table.go at NewDiffTable and trace how the before and after schemas are selected. Add a regression test for a diff where columns are added or deleted, and confirm the dolt_diff schema represents both versions accurately.

Written by the indexing model from the issue text.