merge, sequencer: switch recursive merges over to ort
More precisely, replace calls to merge_recursive() with
merge_ort_recursive().
Also change t7615 to quit calling out recursive; it is not needed
anymore, and we are in fact using ort now.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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