checkout: don't crash on file checkout before running post-checkout hook

checkout: don't crash on file checkout before running post-checkout hook

In the case of

	git init
	echo exit >.git/hooks/post-checkout
	chmod +x .git/hooks/post-checkout
	touch foo
	git add foo
	rm foo
	git checkout -- foo

git-checkout resulted in a Segmentation fault, because there is no new
branch set for the post-checkout hook.

This patch makes use of the null SHA as it is set for the old branch.

While at it, I removed the xstrdup() around the sha1_to_hex(...) calls
in builtin-checkout.c/post_checkout_hook() because sha1_to_hex()
uses four buffers for the hex-dumped SHA and we only need two.
(Duplicating one buffer is only needed if we need more than four.)

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>