Don't autocomplete in command line for now.

For now, we would end up with bad completions anyway
because we always use a ViWordCompletion when
autocompleting.
Set the range properly when completing aggressively.

When we complete aggressively, we go through a separate
return flow if there is only one completion available. We
weren't setting up the range properly for that situation.
Properly update _prefixLength/_range for partial completion.

Partial completion fills in a common prefix of multiple
completions. It's typically triggered by the tab key. We had
changed how we computed the range in those situations,
and stopped updating prefixLength properly. This broke
range computations and therefore insertion in these cases.