patch 8.0.1204: a QuitPre autocommand may get the wrong file name

Problem:    A QuitPre autocommand may get the wrong file name.
Solution:   Pass the buffer being closed to apply_autocmds(). (Rich Howe)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 593d1cc..f5a2289 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -779,3 +779,17 @@
   " check that bufinfo doesn't contain a pointer to freed memory
   call test_garbagecollect_now()
 endfunc
+
+func Test_QuitPre()
+  edit Xfoo
+  let winid = win_getid(winnr())
+  split Xbar
+  au! QuitPre * let g:afile = expand('<afile>')
+  " Close the other window, <afile> should be correct.
+  exe win_id2win(winid) . 'q'
+  call assert_equal('Xfoo', g:afile)
+ 
+  unlet g:afile
+  bwipe Xfoo
+  bwipe Xbar
+endfunc