merge-ort-wrappers: new convience wrappers to mimic the old merge API

merge-ort-wrappers: new convience wrappers to mimic the old merge API

There are a few differences between the new API in merge-ort and the old
API in merge-recursive.  While the new API is more flexible, it might
feel like more work at times than the old API.  merge-ort-wrappers
creates two convenience wrappers taking the exact same arguments as the
old merge_trees() and merge_recursive() functions and implements them
via the new API.  This makes converting existing callsites easier, and
serves to highlight some of the differences in the API.

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