Fix ignore case when diffing recursively

[thj]

Summary

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..

Test Plan

mkdir a
mkdir b

echo "hello" > a/foo
echo "hello" > b/FOO

set -o xtrace

gdiff -u -r --ignore-file-name-case a b
diff -u -r --ignore-file-name-case a b
$diffcmd -u -r --ignore-file-name-case a b

before diff will report:

diff: b/foo: No such file or directory

[bapt]

Would be nice to add unit test

[emaste]

When !ignore_file_name_case will dp2->d_name be equal to dp->d_name?

[thj]

• Add tests for ignore case
• Correct path passed to diffit when using Nflag or Pflag