Adjust for the new way of enabling the default post-update hook

Adjust for the new way of enabling the default post-update hook

The post-update hook, which is required to be enabled in order for
the repository to be accessible over HTTP, is not enabled by
chmod a+x anymore, but instead by dropping the .sample suffix.

This patch emphasizes this change in the release notes (since
I believe this is rather noticeable backwards-incompatible change).
It also adjusts the documentation which still described the old way
and fixes t/t5540-http-push.sh, which was broken for 1.5 month
but apparently noone ever runs this test.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>