Trigger FugitiveChanged after writing stage buffer

I deliberately omitted this event when adding FugitiveChanged, because I
figured it made sense for consumers to instead tap into BufWritePost to
see which file changed exactly.  This would enable, say, vim-gitgutter,
to only refresh signs for the file that actually changed, rather
than for every file in the repository.  In practice, however,
vim-gitgutter doesn't even bother with "in the repository", let alone
the exact file, opting instead to refresh every loaded buffer.  I give
up.

Resolves: https://github.com/tpope/vim-fugitive/issues/1819

autoload/fugitive.vim

@@ -2752,6 +2752,7 @@ function! fugitive#FileWriteCmd(...) abort
       if exists('#' . autype . 'WritePost')
         execute s:DoAutocmd(autype . 'WritePost ' . s:fnameescape(amatch))
       endif
+      exe s:DoAutocmdChanged(dir)
       return ''
     else
       return 'echoerr '.string('fugitive: '.error)