diffcore-rename: provide basic implementation of idx_possible_rename()

diffcore-rename: provide basic implementation of idx_possible_rename()

Add a new struct dir_rename_info with various values we need inside our
idx_possible_rename() function introduced in the previous commit.  Add a
basic implementation for this function showing how we plan to use the
variables, but which will just return early with a value of -1 (not
found) when those variables are not set up.

Future commits will do the work necessary to set up those other
variables so that idx_possible_rename() does not always return -1.

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