http-push: clean up objects list

http-push: clean up objects list

In http-push's get_delta(), we generate a list of pending objects by
recursively processing trees and blobs, adding them to a linked list.
And then we iterate over the list, adding a new request for each
element.

But since we iterate using the list head pointer, at the end it is NULL
and all of the actual list structs have been leaked.

We can fix this either by using a separate iterator and then calling
object_list_free(), or by just freeing as we go. I picked the latter,
just because it means we continue to shrink the list as we go, though
I'm not sure it matters in practice (we call add_send_request() in the
loop, but I don't think it ever looks at the global objects list
itself).

This fixes several leaks noticed in t5540.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>