packfile.c: remove unnecessary prepare_packed_git() call

In 454ea2e4d7 (treewide: use get_all_packs, 2018-08-20) we converted
existing calls to both:

  - get_packed_git(), as well as
  - the_repository->objects->packed_git

, to instead use the new get_all_packs() function.

In the instance that this commit addresses, there was a preceding call
to prepare_packed_git(), which dates all the way back to 660c889e46
(sha1_file: add for_each iterators for loose and packed objects,
2014-10-15) when its caller (for_each_packed_object()) was first
introduced.

This call could have been removed in 454ea2e4d7, since get_all_packs()
itself calls prepare_packed_git(). But the translation in 454ea2e4d7 was
(to the best of my knowledge) a find-and-replace rather than inspecting
each individual caller.

Having an extra prepare_packed_git() call here is harmless, since it
will notice that we have already set the 'packed_git_initialized' field
and the call will be a noop. So we're only talking about a few dozen CPU
cycles to set up and tear down the stack frame.

But having a lone prepare_packed_git() call immediately before a call to
get_all_packs() confused me, so let's remove it as redundant to avoid
more confusion in the future.

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

packfile.c

@@ -2220,7 +2220,6 @@ int for_each_packed_object(struct repository *repo, each_packed_object_fn cb,
 	int r = 0;
 	int pack_errors = 0;
 
-	prepare_packed_git(repo);
 	for (p = get_all_packs(repo); p; p = p->next) {
 		if ((flags & FOR_EACH_OBJECT_LOCAL_ONLY) && !p->pack_local)
 			continue;