merge-tree: rename merge_trees() to trivial_merge_trees()

merge-tree: rename merge_trees() to trivial_merge_trees()

merge-recursive.h defined its own merge_trees() function, different than
the one found in builtin/merge-tree.c.  That was okay in the past, but
we want merge-tree to be able to use the merge-ort functions, which will
end up including merge-recursive.h.  Rename the function found in
builtin/merge-tree.c to avoid the conflict.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>