pretty_print_commit(): do not append notes message

pretty_print_commit(): do not append notes message

The only case pretty_print_commit() appends notes message to the log
message taken from the commit is when show_log() calls it with the
notes_message field set, and the output format is not the userformat
(i.e. when substituting "%N").  No other users of this function sets
this field in the pretty_print_context, as can be easily verified in
the previous step.

Hoist the code to append the notes message to the caller.

Up to this point, no functionality change is intended.

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