Make FugitiveChanged event aware of :Git result

If g:fugitive_result is defined during the User FugitiveChanged event,
one can trigger a custom behavior based on the arguments in .args or the
output in .file.

References https://github.com/tpope/vim-fugitive/pull/1015

plugin/fugitive.vim

@@ -22,6 +22,8 @@ function! FugitiveGitDir(...) abort
     return getbufvar(a:1, 'git_dir')
   elseif type(a:1) == type('')
     return substitute(s:Slash(a:1), '/$', '', '')
+  elseif type(a:1) == type({})
+    return get(a:1, 'git_dir', '')
   else
     return ''
   endif