Ship sample hooks with .sample suffix

Ship sample hooks with .sample suffix

We used to mark hooks we ship as samples by making them unexecutable, but
some filesystems cannot tell what is executable and what is not.

This makes it much more explicit.  The hooks are suffixed with .sample
(but now are made executable), so enabling it is still one step operation
(instead of "chmod +x $hook", you would do "mv $hook.sample $hook") but
now they won't get accidentally enabled on systems without executable bit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>