merge-ort: port merge_start() from merge-recursive

merge_start() basically does a bunch of sanity checks, then allocates
and initializes opt->priv -- a struct merge_options_internal.

Most of the sanity checks are usable as-is.  The
allocation/intialization is a bit different since merge-ort has a very
different merge_options_internal than merge-recursive, but the idea is
the same.

The weirdest part here is that merge-ort and merge-recursive use the
same struct merge_options, even though merge_options has a number of
fields that are oddly specific to merge-recursive's internal
implementation and don't even make sense with merge-ort's high-level
design (e.g. buffer_output, which merge-ort has to always do).  I reused
the same data structure because:
  * most the fields made sense to both merge algorithms
  * making a new struct would have required making new enums or somehow
    externalizing them, and that was getting messy.
  * it simplifies converting the existing callers by not having to
    have different code paths for merge_options setup.

I also marked detect_renames as ignored.  We can revisit that later, but
in short: merge-recursive allowed turning off rename detection because
it was sometimes glacially slow.  When you speed something up by a few
orders of magnitude, it's worth revisiting whether that justification is
still relevant.  Besides, if folks find it's still too slow, perhaps
they have a better scaling case than I could find and maybe it turns up
some more optimizations we can add.  If it still is needed as an option,
it is easy to add later.

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

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