commit re-encoding: fix confusion between no and default conversion.

commit re-encoding: fix confusion between no and default conversion.

Telling the git-log family not to do any character conversion is
done with --encoding=none, which sets log_output_encoding to an
empty string.  However, logmsg_reencode() confused this with
log_output_encoding and commit_encoding set to NULL.  The latter
means we should use the default encoding (i.e. utf-8).

Signed-off-by: Junio C Hamano <junkio@cox.net>