builtin/pack-objects.c: don't leak memory via arguments

builtin/pack-objects.c: don't leak memory via arguments

When constructing arguments to pass to setup_revision(), pack-objects
only frees the memory used by that array after calling
get_object_list().

Ensure that we call strvec_clear() whether or not we use the arguments
array by cleaning up whenever we exit the function (and rewriting one
early return to jump to a label which frees the memory and then
returns).

We could avoid setting this array up altogether unless we are in the
if-else block that calls get_object_list(), but setting up the argument
array is intermingled with lots of other side-effects, e.g.:

    if (exclude_promisor_objects) {
      use_internal_rev_list = 1;
      fetch_if_missing = 0;
      strvec_push(&rp, "--exclude-promisor-objects");
    }

So it would be awkward to check exclude_promisor_objects twice: first to
set use_internal_rev_list and fetch_if_missing, and then again above
get_object_list() to push the relevant argument onto the array.

Instead, leave the array's construction alone and make sure to free it
unconditionally.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>