merge-ort: enable diff-algorithms other than histogram

merge-ort: enable diff-algorithms other than histogram

The ort merge strategy has always used the histogram diff algorithm.
The recursive merge strategy, in contrast, defaults to the myers
diff algorithm, while allowing it to be changed.

Change the ort merge strategy to allow different diff algorithms, by
removing the hard coded value in merge_start() and instead just making
it a default in init_merge_options().  Technically, this also changes
the default diff algorithm for the recursive backend too, but we're
going to remove the final callers of the recursive backend in the next
two commits.

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