pack-objects: free packing_data in more places

The pack-objects internals use a packing_data struct to track what
objects are part of the pack(s) being formed.

Since these structures contain allocated fields, failing to
appropriately free() them results in a leak. Plug that leak by
introducing a clear_packing_data() function, and call it in the
appropriate spots.

This is a fairly straightforward leak to plug, since none of the callers
expect to read any values or have any references to parts of the address
space being freed.

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

builtin/pack-objects.c

@@ -4522,6 +4522,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			   reuse_packfile_objects);
 
 cleanup:
+	clear_packing_data(&to_pack);
 	list_objects_filter_release(&filter_options);
 	strvec_clear(&rp);