merge-ort: add a few includes

merge-ort: add a few includes

Include blob.h for definition of blob_type, and commit-reach.h for
declarations of get_merge_bases() and in_merge_bases().  While none of
these are used yet, we want to avoid cross-dependencies in the next
three series of patches for merge-ort and merge them at the end; adding
these "#include"s now avoids textual conflicts.

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