patch 8.2.2228: Vim9: cannot use ":e #" because # starts a comment

Problem:    Vim9: cannot use ":e #" because # starts a comment.
Solution:   Support using %% instead of #.

src/testdir/test_vim9_cmd.vim

@@ -25,6 +25,53 @@ def Test_edit_wildcards()
   CheckDefFailure(['edit `="foo"'], 'E1083:')
 enddef
 
+def Test_expand_alternate_file()
+  var lines =<< trim END
+    edit Xfileone
+    var bone = bufnr()
+    edit Xfiletwo
+    var btwo = bufnr()
+    edit Xfilethree
+    var bthree = bufnr()
+
+    edit #
+    assert_equal(bthree, bufnr())
+    edit %%
+    assert_equal(btwo, bufnr())
+    edit %% # comment
+    assert_equal(bthree, bufnr())
+    edit %%yy
+    assert_equal('Xfiletwoyy', bufname())
+
+    exe "edit %%" .. bone
+    assert_equal(bone, bufnr())
+    exe "edit %%" .. btwo .. "xx"
+    assert_equal('Xfiletwoxx', bufname())
+
+    next Xfileone Xfiletwo Xfilethree
+    assert_equal('Xfileone', argv(0))
+    assert_equal('Xfiletwo', argv(1))
+    assert_equal('Xfilethree', argv(2))
+    next %%%zz
+    assert_equal('Xfileone', argv(0))
+    assert_equal('Xfiletwo', argv(1))
+    assert_equal('Xfilethreezz', argv(2))
+
+    v:oldfiles = ['Xonefile', 'Xtwofile']
+    edit %%<1
+    assert_equal('Xonefile', bufname())
+    edit %%<2
+    assert_equal('Xtwofile', bufname())
+    assert_fails('edit %%<3', 'E684:')
+
+    edit Xfileone.vim
+    edit Xfiletwo
+    edit %%:r
+    assert_equal('Xfileone', bufname())
+  END
+  CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_global_backtick_expansion()
   new
   setline(1, 'xx')