This didn't do all the syntax checks, so something like
fish -c 'echo foo; and $status'
complained of a missing command `0` (i.e. $status), and
fish -c 'echo foo | exec grep'
hit an assert!
So we do what read_ni does, parse each command into an ast, run
parse_util_detect_errors on it if it worked and then eval the ast.
It is possible to do this neater by modifying parser::eval, but I
can't find where.
