Many scripts, most notably gitk, rely on output from the log family of
command not to be molested by random user configuration.  This is
especially true when --pretty=raw is given.

Just like we disable notes output unless the command line explicitly
asks for --show-notes, disable the decoration code unless --decorate is
given explicitly from the command line and --pretty or --oneline is
given.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>