mingw_rename: do support directory renames

In 391bcea (compat/mingw: support POSIX semantics for atomic
renames, 2024-10-27), we taught the `mingw_rename()` function to respect
POSIX semantics, but we did so only as a fallback after `_wrename()`
fails.

This hid a bug in the implementation that was not caught by Git's test
suite: The `CreateFileW()` function _can_ open handles to directories,
but not when asked to use the `FILE_ATTRIBUTE_NORMAL` flag, as that flag
only is allowed for files.

Let's fix this by using the common `FILE_FLAG_BACKUP_SEMANTICS` flag
that can be used for opening handles to directories, too.

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

compat/mingw.c

@@ -2273,7 +2273,7 @@ int mingw_rename(const char *pold, const char *pnew)
 
 		old_handle = CreateFileW(wpold, DELETE,
 					 FILE_SHARE_WRITE | FILE_SHARE_READ | FILE_SHARE_DELETE,
-					 NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
+					 NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
 		if (old_handle == INVALID_HANDLE_VALUE) {
 			errno = err_win_to_posix(GetLastError());
 			return -1;