merge-ort: step 2 of tree writing -- function to create tree object

merge-ort: step 2 of tree writing -- function to create tree object

Create a new function, write_tree(), which will take a list of
basenames, modes, and oids for a single directory and create a tree
object in the object-store.  We do not yet have just basenames, modes,
and oids for just a single directory (we have a mixture of entries from
all directory levels in the hierarchy) so we still die() before the
current call to write_tree(), but the next patch will rectify that.

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