unable to remove oddly-named directory

[Jeremy Drake]

Backstory: rust's test suite makes an oddly-named directory as part of a
test:
https://github.com/rust-lang/rust/blob/921645c737f1d6d107a0a10ca5ee129d364dcd7a/tests/run-make/non-unicode-in-incremental-dir/rmake.rs

When trying to clean up after a rust build/test with rm -rf, it results in
a "Directory not empty" error.

Thankfully, this can be simply reproduced with the following two bash
commands (on cygwin 3.5.3):

mkdir -p foo/$'\uD800'
rm -rf foo

This fails with: rm: cannot remove 'foo': Directory not empty
when it should succeed.