merge-ort: add die-not-implemented stub handle_content_merge() function
This simplistic and weird-looking patch is here to facilitate future
patch submissions. Adding this stub allows rename detection code to
reference it in one patch series, while a separate patch series can
define the implementation, and then both series can merge cleanly and
work nicely together at that point.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>