merge-recursive: remove useless parameter in merge_trees()
merge_trees() took a results parameter that would only be written when
opt->call_depth was positive, which is never the case now that
merge_trees_internal() has been split from merge_trees(). Remove the
misleading and unused parameter from merge_trees().
While at it, add some comments explaining how the output of
merge_trees() and merge_recursive() differ.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>