merge-ort: add a new toplevel_dir field

merge-ort: add a new toplevel_dir field

Due to the string-equality-iff-pointer-equality requirements placed on
merged_info.directory_name, apply_directory_rename_modifications() will
need to have access to the exact toplevel directory name string pointer
and can't just use a new empty string.  Store it in a field that we can
use.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>