[review] Allow re-assigning to convenience variables

[Pedro Alves]

LGTM.

> +    gdb_test "print \$arr = { 1, 2, 3}" \

> +    gdb_test "print \$arr = { 0, 1, 2, 3}" \

Noticed the spurious space after '{' but not before '}'.

Thanks,
Pedro Alves