pipeStages splits command with >> incorrectly

[myan9]

For commands that have >>, splitIntoPipeStages splits the command as redirecting to a file with prefix '> '

[starpit]

It's not just >>, but also these (and other related) forms:

>
>>
>&
2>1

[starpit]

@myan9 suggests that at the very least, we can just not parse >> as a redirect in the pipe stage parser

[starpit]

I suggest we split this into two pieces: one is the parsing issue. Second, in repl/exec, we always use vfs fwrite even if the redirect is an append, e.g. >>. The latter will require some updates to the fwrite api, so I suggest we handle that separately.