log_ref_setup(): manage the name of the reflog file internally

log_ref_setup(): manage the name of the reflog file internally

Instead of writing the name of the reflog file into a strbuf that is
supplied by the caller but not needed there, write it into a local
temporary buffer and remove the strbuf parameter entirely.

And while we're adjusting the function signature, reorder the arguments
to move the input parameters before the output parameters.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>