This cleans up and simplifies the API by passing the command name as the
first argument. Previously the command name was specified explicitly,
but was still included in the argv.
(cherry picked from commit 056a43e1a615080cf9ba405be0538a6a979c821e)
This is essentially the same as calling COMMAND GETKEYS but provides a
more efficient interface that can be used in every context (i.e. not a
Redis command).
(cherry picked from commit 7d117d7591656e947f526f5d5f8a022b88b38ad9)