pretty: Initialize notes if %N is used

pretty: Initialize notes if %N is used

When using git log --pretty='%N' without an explicit --show-notes, git
would segfault. This patches fixes this behaviour by loading the needed
notes datastructures if --pretty is used and the format contains %N.
When --pretty='%N' is used together with --no-notes, %N won't be
expanded.

This is an extension to a proposed patch by Jeff King.

Signed-off-by: Johannes Gilger <heipei@hackvalue.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>