Don't do extra query when -a not used

There was a mistake in the check to see if the -a/--all-from query
needed to be issued.  get_changes() was being given a list with a
single item of an empty string.  It expects an empty list if the
option wasn't specified.
Ensure reviews are not duplicated in output.

There are two queries issued to gerrit when the -a/--all-from option
is specified.  A review may appear in the results of both queries.
Make sure that it is only included in the output once.