mingw: emulate `stat()` a little more faithfully

mingw: emulate `stat()` a little more faithfully

When creating directories via `safe_create_leading_directories()`, we
might encounter an already-existing directory which is not
readable by the current user. To handle that situation, Git's code calls
`stat()` to determine whether we're looking at a directory.

In such a case, `CreateFile()` will fail, though, no matter what, and
consequently `mingw_stat()` will fail, too. But POSIX semantics seem to
still allow `stat()` to go forward.

So let's call `mingw_lstat()` to the rescue if we fail to get a file
handle due to denied permission in `mingw_stat()`, and fill the stat
info that way.

We need to be careful to not allow this to go forward in case that we're
looking at a symbolic link: to resolve the link, we would still have to
create a file handle, and we just found out that we cannot. Therefore,
`stat()` still needs to fail with `EACCES` in that case.

This fixes https://github.com/git-for-windows/git/issues/2531.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>