'''```
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`