merge_trees(): ensure that the callers release output buffer

merge_trees(): ensure that the callers release output buffer

The recursive merge machinery accumulates its output in an output
buffer, to be flushed at the end of merge_recursive(). At this point,
we forgot to release the output buffer.

When calling merge_trees() (i.e. the non-recursive part of the recursive
merge) directly, the output buffer is never flushed because the caller
may be merge_recursive() which wants to flush the output itself.

For the same reason, merge_trees() cannot release the output buffer: it
may still be needed.

Forgetting to release the output buffer did not matter much when running
git-checkout, or git-merge-recursive, because we exited after the
operation anyway. Ever since cherry-pick learned to pick a commit range,
however, this memory leak had the potential of becoming a problem.

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