The diff options already know how to print the output anywhere else
than stdout. The same is needed for log output in general, e.g.
when writing patches to files in `git format-patch`. Let's allow
users to use log_tree_commit() *without* changing global state via
freopen().
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>