merge-ort: flesh out implementation of handle_content_merge()

merge-ort: flesh out implementation of handle_content_merge()

This implementation is based heavily on merge_mode_and_contents() from
merge-recursive.c, though it has some fixes for recursive merges (i.e.
when call_depth > 0), and has a number of changes throughout based on
slight differences in data structures and in how the functions are
called.

It is, however, based on two new helper functions -- merge_3way() and
merge_submodule -- for which we only provide die-not-implemented stubs
at this point.  Future commits will add implementations of these
functions.

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