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>