refs: work around network caching on Windows

Network shares sometimes use aggressive caching, in which case a
just-created directory might not be immediately visible to Git.

One symptom of this scenario is the following error:

	$ 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 in all Windows setups. 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/

the following commands worked around it:

	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 culprit is that `GetFileAttributesExW()` that is called from
`mingw_lstat()` can raise an error `ERROR_PATH_NOT_FOUND`, which is
translated to `ENOTDIR`, as opposed to `ENOENT` as expected on Linux.

Therefore, when trying to read a ref, let's allow `ENOTDIR` in addition
to `ENOENT` to indicate that said ref is missing.

This fixes #3727

Signed-off-by: Pierre Garnier <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: pgrmega

refs/files-backend.c

@@ -381,7 +381,7 @@ static int read_ref_internal(struct ref_store *ref_store, const char *refname,
 	if (lstat(path, &st) < 0) {
 		int ignore_errno;
 		myerr = errno;
-		if (myerr != ENOENT || skip_packed_refs)
+		if ((myerr != ENOENT && myerr != ENOTDIR) || skip_packed_refs)
 			goto out;
 		if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
 				      referent, type, &ignore_errno)) {

refs/packed-backend.c

@@ -480,7 +480,7 @@ static int load_contents(struct snapshot *snapshot)
 
 	fd = open(snapshot->refs->path, O_RDONLY);
 	if (fd < 0) {
-		if (errno == ENOENT) {
+		if (errno == ENOENT || errno == ENOTDIR) {
 			/*
 			 * This is OK; it just means that no
 			 * "packed-refs" file has been written yet,