Fix "git clone http://$URL" to check out the worktree when asked

Fix "git clone $URL" to check out the worktree when asked

The builtin-clone now does the http commit walking and the tree unpacking
in the same process, and the commit walker leaves the in-core objects in a
funny state.  When forgetting the data read from the tree object, the
object should be marked "not parsed yet" for later users.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>