make cscope.out connect automatic and correctly when a file in subdirectories open

**Puspose**
To fix a problem I ever met. It's description is below.

**Problem Description**
If the is a directory tree like this:

```
D:\CSCOPE_TEST
│  cscope.out
│  main.cpp
│
└─headers
        functions.cpp
        functions.h
```

If I work with original cscope_macros.vim plugin, when I open `main.cpp`, it will connect cscope.out automatic and correctly. But if I switch to files in subdir `headers` use the same vim instance, cscope won't work. And if I open `functsions.cpp` or `functions.h` with vim, cscope won't work neither.