lstat(mingw): correctly detect ENOTDIR scenarios

Files' attributes can indicate more than just whether they are files or
directories. It was reported in Git for Windows that on certain network
shares, this led to a nasty problem trying to create tags:

	$ git tag -a -m "automatic tag creation"  test_dir/test_tag
	fatal: cannot lock ref 'refs/tags/test_dir/test_tag': unable to resolve reference 'refs/tags/test_dir/test_tag': Not a directory

Note: This does not necessarily happen with all types of network shares.
One setup where it _did_ happen is a Windows Server 2019 VM, and as
hinted in

	http://woshub.com/slow-network-shared-folder-refresh-windows-server/

in the indicated instance the following commands worked around the bug:

	Set-SmbClientConfiguration -DirectoryCacheLifetime 0
	Set-SmbClientConfiguration -FileInfoCacheLifetime 0
	Set-SmbClientConfiguration -FileNotFoundCacheLifetime 0

This would impact performance negatively, though, as it essentially
turns off all caching, therefore we do not want to require users to do
that just to be able to use Git on Windows.

The underlying bug is in the code added in 4b0abd5 (mingw: let
lstat() fail with errno == ENOTDIR when appropriate, 2016-01-26) that
emulates the POSIX behavior where `lstat()` should return `ENOENT` if
the file or directory simply does not exist but could be created, and
`ENOTDIR` if there is no file or directory nor could there be because a
leading path already exists and is not a directory.

In that code, the return value of `GetFileAttributesW()` is interpreted
as an enum value, not as a bit field, so that a perfectly fine leading
directory can be misdetected as "not a directory".

As a consequence, the `read_refs_internal()` function would return
`ENOTDIR`, suggesting not only that the tag in the `git tag` invocation
above does not exist, but that it cannot even be created.

Let's fix the code so that it interprets the return value of the
`GetFileAtrtibutesW()` call correctly.

This fixes git-for-windows#3727

Reported-by: Pierre Garnier <pgarnier@mega.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -471,8 +471,8 @@ static int has_valid_directory_prefix(wchar_t *wfilename)
 		wfilename[n] = L'\0';
 		attributes = GetFileAttributesW(wfilename);
 		wfilename[n] = c;
-		if (attributes == FILE_ATTRIBUTE_DIRECTORY ||
-				attributes == FILE_ATTRIBUTE_DEVICE)
+		if (attributes &
+		    (FILE_ATTRIBUTE_DIRECTORY | FILE_ATTRIBUTE_DEVICE))
 			return 1;
 		if (attributes == INVALID_FILE_ATTRIBUTES)
 			switch (GetLastError()) {