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 a94ddb27fe919d60c598c2403b230b27c6e3a11c)
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 9e58b52a19fed12c4e9fbe120ec0d4baf9c2bc28)