`dolt diff --stat -r sql` is unimplemented

[jycor]

Before, we used to print tabular output for dolt diff --stat -r sql. Now, we just throw an unsupported error. There's a skipped bats test in sql-diff.bats

Repro:

dolt init
dolt sql -q "create table t (i int);"
dolt diff --stat -r sql

Added here: https://github.com/dolthub/dolt/pull/7810