line-log: respect diffopt's configured output file stream

line-log: respect diffopt's configured output file stream

The diff machinery can optionally output to a file stream other than
stdout, by overriding diffopt.file. In such a case, the rest of the
log tree machinery should also write to that stream.

Currently, there is no user of the line level log that wants to
redirect output to a file. Therefore, one might argue that it is
superfluous to support that now. However, it is better to be
consistent now, rather than to face hard-to-debug problems later.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>