vsh: Simplify condition for calling completer callback

vsh: Simplify condition for calling completer callback

The way we currently call completer callbacks is that if we've
found --option that user wants to complete value for and it has
callback set then the callback is called.

And just before that, if no --option to have the value completed
is found or is found and is of boolean type then a list of
--option is generated (for given command).

But these two conditions can never be true at the same time
because boolean type of --options do not accept values. Therefore
the calling of completer callback can be promoted onto the same
level as the --option list generation.

This means that merging of two lists can be dropped to and
completer callback can store its retval directly into @list (but
as shown earlier one of the string lists to merge is always
empty).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>