[Nmh-workers] pick nit

[Jerrad Pierce]

I recently discovered what seems to be a wart in pick,
it resets Previous-Sequence to *all*. It does not set
it to match the returned results, nor does it leave the
sequence unmoleated. This unespected behavior resulted
in a very nasty surprise when I ended up removing all
of my messages after invoking pick! (I usually use an
alias that does 'scan `pick`', and don't encounter this)

I think onw of the other behaviors above would be best,
but worst case perhaps MH should seek user confirmation
when doing something to a group of messages equivalent
to "all" if pick is deemed too ossified to change.

-Jerrad