Set the local, not global, value of cursorbind

From src/option.c:

```c
/*
 * Options local to a window have a value local to a buffer and global to all
 * buffers.  Indicate this by setting "var" to VAR_WIN.
 */
#define VAR_WIN ((char_u *)-1)
…
    {"cursorbind",  "crb",  P_BOOL|P_VI_DEF,
                            (char_u *)VAR_WIN, PV_CRBIND,
                                      ^^^^^^^
```

Steps to reproduce:

1. `:Gstatus` with at least one modified file
2. `D` on that file to open two diff windows
3. `:only` from one of the windows
4. `:Gstatus` again
5. `D` to diff again
6. Move the cursor in one of the diff windows and watch the `Gstatus` window scroll with the cursor