specify explicit "--pretty=medium" with `git log/show/whatchanged`

specify explicit "--pretty=medium" with `git log/show/whatchanged`

The following patch will introduce a new configuration variable,
"format.pretty", from then on the pretty format without specifying
"--pretty" might not be the default "--pretty=medium", it depends on
the user's config. So all kinds of Shell/Perl/Emacs scripts that needs
the default medium pretty format must specify it explicitly.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>