mingw_open_existing: handle directories better

mingw_open_existing: handle directories better

CreateFileW() requires FILE_FLAG_BACKUP_SEMANTICS to create a directory
handle [1] and errors out with ERROR_ACCESS_DENIED without this flag.
Fall back to accessing Directory handles this way.

[1] https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilew#directories

This fixes https://github.com/git-for-windows/git/issues/5068

Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>