diff: fix failed compare when ignoring file case

With --ignore-file-name-case we need to compare files regardless of
case. We need to propigate both names down to diffit so we can look up
the correct file when the names differ based on case, otherwise we try
to look up the file using the case from the a tree which might not be
discoverable if its case is different in the b tree.

Reviewed by: bapt
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D34201