patch 8.0.0205: wrong behavior after :undojoin

Problem:    After :undojoin some commands don't work properly, such as :redo.
            (Matthew Malcomson)
Solution:   Don't set curbuf->b_u_curhead. (closes #1390)

Author: brammool

src/testdir/test_undo.vim

@@ -176,7 +176,17 @@ func Test_undojoin()
   call assert_equal(['aaaa', 'bbbb', 'cccc'], getline(2, '$'))
   call feedkeys("u", 'xt')
   call assert_equal(['aaaa'], getline(2, '$'))
-  close!
+  bwipe!
+endfunc
+
+func Test_undojoin_redo()
+  new
+  call setline(1, ['first line', 'second line'])
+  call feedkeys("ixx\<Esc>", 'xt')
+  call feedkeys(":undojoin | redo\<CR>", 'xt')
+  call assert_equal('xxfirst line', getline(1))
+  call assert_equal('second line', getline(2))
+  bwipe!
 endfunc
 
 func Test_undo_write()

src/undo.c

@@ -3136,11 +3136,8 @@ ex_undojoin(exarg_T *eap UNUSED)
     if (get_undolevel() < 0)
 	return;		    /* no entries, nothing to do */
     else
-    {
-	/* Go back to the last entry */
-	curbuf->b_u_curhead = curbuf->b_u_newhead;
-	curbuf->b_u_synced = FALSE;  /* no entries, nothing to do */
-    }
+	/* Append next change to the last entry */
+	curbuf->b_u_synced = FALSE;
 }
 
 /*

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    205,
 /**/
     204,
 /**/