Trigger User FugitiveChanged on events that change repository

Note that in the context of this event, Fugitive functions will use the
changed repository, which might not be the same as the current buffer's
repository.  Pass bufnr() as the optional dir argument to force the
current buffer.

Closes https://github.com/tpope/vim-fugitive/issues/458

plugin/fugitive.vim

@@ -10,6 +10,9 @@ let g:loaded_fugitive = 1
 
 function! FugitiveGitDir(...) abort
   if !a:0 || type(a:1) == type(0) && a:1 < 0
+    if exists('g:fugitive_event')
+      return g:fugitive_event
+    endif
     let dir = get(b:, 'git_dir', '')
     if empty(dir) && (empty(bufname('')) || &buftype =~# '^\%(nofile\|acwrite\|quickfix\|prompt\)$')
       return FugitiveExtractGitDir(getcwd())