prune_remote(): initialize both delete_refs lists in a single loop

prune_remote(): initialize both delete_refs lists in a single loop

Also free them together at the end of the function.

In a moment, the array version will become redundant. Managing them
together makes later steps more obvious.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>