[RFC] vim-patch:7.4.274

```
Problem:  When doing ":update" just before running an external command that
          changes the file, the timestamp may be unchanged and the file
          is not reloaded.
Solution: Also check the file size.
```

https://code.google.com/p/vim/source/detail?r=1ee3fc5b40ae94c2a7fc5a62bca38d4f730f9bb2