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()` for 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 git-for-windows#2531.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1019,7 +1019,19 @@ int mingw_stat(const char *file_name, struct stat *buf)
 			FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
 			OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
 	if (hnd == INVALID_HANDLE_VALUE) {
-		errno = err_win_to_posix(GetLastError());
+		DWORD err = GetLastError();
+
+		if (err == ERROR_ACCESS_DENIED &&
+		    !mingw_lstat(file_name, buf) &&
+		    !S_ISLNK(buf->st_mode))
+			/*
+			 * POSIX semantics state to still try to fill
+			 * information, even if permission is denied to create
+			 * a file handle.
+			 */
+			return 0;
+
+		errno = err_win_to_posix(err);
 		return -1;
 	}
 	result = get_file_info_by_handle(hnd, buf);