patch 8.2.2166: auto format doesn't work when deleting text

Problem:    Auto format doesn't work when deleting text.
Solution:   Make "x" trigger auto format. (closes #7504)

src/testdir/test_textformat.vim

@@ -934,6 +934,18 @@ func Test_fo_a_w()
   call assert_equal('g uu uu ', getline(1)[-8:])
   call assert_equal(['uu. foo'], getline(2, '$'))
 
+  " using backspace or "x" triggers reformat
+  call setline(1, ['1 2 3 4 5 ', '6 7 8 9'])
+  set tw=10
+  set fo=taw
+  set bs=indent,eol,start
+  exe "normal 1G4la\<BS>\<BS>\<Esc>"
+  call assert_equal(['1 2 4 5 6 ', '7 8 9'], getline(1, 2))
+  exe "normal f4xx"
+  call assert_equal(['1 2 5 6 7 ', '8 9'], getline(1, 2))
+
+  set tw=0
+  set fo&
   %bw!
 endfunc