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

[James Cor]

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: #7810