enable buffer-specifc settings

While saving a `fugitive://` buffer I get the error shown in the screenshot.

![screenshot](http://i.imgur.com/jGDYH74.png)

I'm submitting this pull request so that it's possible to disable autotag for specific filenames, by using an autocmd like the following one: `autocmd BufEnter fugitive://**//[0-3]/** let b:autotagDisabled=1`