diffcore-rename: add a mapping of destination names to their indices

diffcore-rename: add a mapping of destination names to their indices

Compute a mapping of full filename to the index within rename_dst where
that filename is found, and store it in idx_map.  idx_possible_rename()
needs this to quickly finding an array entry in rename_dst given the
pathname.

While at it, add placeholder initializations for dir_rename_count and
dir_rename_guess; these will be more fully populated in subsequent
commits.

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