[RFC] vim-patch:7.4.456

```
Problem:  'backupcopy' is global, cannot write only some
          files in a different way.

Solution: Make 'backupcopy' global-local. (Christian Brabandt)
```

https://code.google.com/p/vim/source/detail?r=54194bd6ed607aa0cec07d8a27d4794573eabba9
doxygen (`///<` for inline documentation)