vim-patch:9.2.0914: diff: undo after :diffget into an empty buffer leaves a line behind (#41181)

Problem:  After :diffget into an empty buffer, undo does not restore the
          empty buffer, the last line stays behind (Narendran
          Gopalakrishnan)
Solution: Include the empty line of the empty buffer in the undo
          information, it is deleted once the first line was obtained
          (Hirohito Higashi).

fixes:  vim/vim#20950
closes: vim/vim#20951

c44f35ca1a

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/old/testdir/test_diffmode.vim

@@ -253,6 +253,24 @@ func Test_diffget_diffput()
   %bwipe!
 endfunc
 
+" Undo after getting lines into an empty buffer must leave it empty again
+func Test_diffget_undo_empty_buffer()
+  enew!
+  diffthis
+  new
+  call setline(1, ['1', '2'])
+  diffthis
+
+  wincmd p
+  normal do
+  call assert_equal(['1', '2'], getline(1, '$'))
+  undo
+  call assert_equal([''], getline(1, '$'))
+
+  windo diffoff
+  %bwipe!
+endfunc
+
 " Test putting two changes from one buffer to another
 func Test_diffput_two()
   new a