diffcore-rename: compute dir_rename_counts in stages

diffcore-rename: compute dir_rename_counts in stages

Compute dir_rename_counts based just on exact renames to start, as that
can provide us useful information in find_basename_matches().  This is
done by moving the code from compute_dir_rename_counts() into
initialize_dir_rename_info(), resulting in it being computed earlier and
based just on exact renames.  Since that's an incomplete result, we
augment the counts via calling update_dir_rename_counts() after each
basename-guide and inexact rename detection match is found.

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