diffcore-rename: move dir_rename_counts into dir_rename_info struct

diffcore-rename: move dir_rename_counts into dir_rename_info struct

This continues the migration of the directory rename detection code into
diffcore-rename, now taking the simple step of combining it with the
dir_rename_info struct.  Future commits will then make dir_rename_counts
be computed in stages, and add computation of dir_rename_guess.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>