merge-recursive: split internal fields into a separate struct

merge_options has several internal fields that should not be set or read
by external callers.  This just complicates the API.  Move them into an
opaque merge_options_internal struct that is defined only in
merge-recursive.c and keep these out of merge-recursive.h.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren