Provide User FugitiveEditor event

I'm not sure if this should trigger every time the buffer is loaded, or
only on the initial split.  Arbitrarily pick the latter for now.

References: https://github.com/tpope/vim-fugitive/issues/1828

autoload/fugitive.vim

@@ -3145,8 +3145,18 @@ function! s:RunEdit(state, tmp, job) abort
   let file = FugitiveVimPath(readfile(sentinel, '', 1)[0])
   exe substitute(a:state.mods, '\<tab\>', '-tab', 'g') 'keepalt split' s:fnameescape(file)
   set bufhidden=wipe
-  let s:edit_jobs[bufnr('')] = [a:state, a:tmp, a:job, sentinel]
+  let bufnr = bufnr('')
+  let s:edit_jobs[bufnr] = [a:state, a:tmp, a:job, sentinel]
   call fugitive#DidChange(a:state.git_dir)
+  if bufnr == bufnr('') && !exists('g:fugitive_event')
+    try
+      let g:fugitive_event = a:state.git_dir
+      let g:fugitive_result = a:state
+      exe s:DoAutocmd('User FugitiveEditor')
+    finally
+      unlet! g:fugitive_event g:fugitive_result
+    endtry
+  endif
   return 1
 endfunction