Trigger BufWritePost event via BufWriteCmd/FileWriteCmd

OK.

What's the reason behind using both BufWriteCmd and FileWriteCmd?
Is writing partial files supported by vim-gnupg?
What does the `'[, ']` range refer to in the `w` ex command being used?

The help says:

    '[  `[                  To the first character of the previously changed      
                            or yanked text.  {not in Vi}                          
                                                                                  
                                                            '] `]                 
    ']  `]                  To the last character of the previously changed or    
                              yanked text.  {not in Vi}