Fix bug when calling GitGutterDisable immediately.

When opening Vim without editing any file, calling GitGutterDisable
would fail because `s:file` was undefined at that point.  The fix breaks
what encapsulation there is, but has the virtue of working.

plugin/gitgutter.vim

@@ -18,6 +18,8 @@ if !exists('g:gitgutter_sign_column_always')
   let g:gitgutter_sign_column_always = 0
 endif
 
+let s:file = ''
+
 function! s:init()
   if !exists('g:gitgutter_initialised')
     call s:define_sign_column_highlight()