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>

builtin-checkout.c

@@ -47,8 +47,10 @@ static int post_checkout_hook(struct commit *old, struct commit *new,
 
 	memset(&proc, 0, sizeof(proc));
 	argv[0] = name;
-	argv[1] = xstrdup(sha1_to_hex(old ? old->object.sha1 : null_sha1));
-	argv[2] = xstrdup(sha1_to_hex(new->object.sha1));
+	argv[1] = sha1_to_hex(old ? old->object.sha1 : null_sha1);
+	argv[2] = sha1_to_hex(new ? new->object.sha1 : null_sha1);
+	/* "new" can be NULL when checking out from the index before
+	   a commit exists. */
 	argv[3] = changed ? "1" : "0";
 	argv[4] = NULL;
 	proc.argv = argv;