Don't set b_u_curhead in ex_undojoin()

vim-patch:8.0.0205
This provides no benefit, and introduces some bugs.

src/nvim/undo.c

@@ -2549,20 +2549,20 @@ static void u_add_time(char_u *buf, size_t buflen, time_t tt)
  */
 void ex_undojoin(exarg_T *eap)
 {
-  if (curbuf->b_u_newhead == NULL)
-    return;                 /* nothing changed before */
+  if (curbuf->b_u_newhead == NULL) {
+    return;                 // nothing changed before
+  }
   if (curbuf->b_u_curhead != NULL) {
     EMSG(_("E790: undojoin is not allowed after undo"));
     return;
   }
-  if (!curbuf->b_u_synced)
-    return;                 /* already unsynced */
-  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 */
+  if (!curbuf->b_u_synced) {
+    return;                 // already unsynced
+  }
+  if (get_undolevel() < 0) {
+    return;                 // no entries, nothing to do
+  } else {
+    curbuf->b_u_synced = false;  // Append next change to last entry
   }
 }
 

test/functional/ex_cmds/undojoin_spec.lua

@@ -0,0 +1,38 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local clear = helpers.clear
+local insert = helpers.insert
+local feed = helpers.feed
+local expect = helpers.expect
+local execute = helpers.execute
+local exc_exec = helpers.exc_exec
+
+describe(':undojoin command', function()
+  before_each(function()
+    clear()
+    insert([[
+    Line of text 1
+    Line of text 2]])
+    execute('goto 1')
+  end)
+  it('joins changes in a buffer', function()
+    execute('undojoin | delete')
+    expect([[
+    Line of text 2]])
+    feed('u')
+    expect([[
+    ]])
+  end)
+  it('does not corrupt undolist when connected with redo', function()
+    feed('ixx<esc>')
+    execute('undojoin | redo')
+    expect([[
+    xxLine of text 1
+    Line of text 2]])
+  end)
+  it('does not raise an error when called twice', function()
+    local ret = exc_exec('undojoin | undojoin')
+    eq(0, ret)
+  end)
+end)