When 'git show' is called without any object it defaults to HEAD. This
has been true since d4ed9793fd ("Simplify common default options setup
for built-in log family.", 2006-04-16).
The SYNOPSIS suggests that the object argument is required. Clarify
that it is not required and note the default.
Signed-off-by: Todd Zullinger <tmz@pobox.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>