ex_getln: Do not crash with :append/:insert/:change

This change effectively disables history for lines inserted using this method. 
Not a big problem since it does not work for them in Vim in first place.

Also solves a bug(?): ex_window() run while in :append mode opens search history 
in Vim for some reason. Now it opens empty cmdline window.

test/functional/ex_cmds/append_spec.lua

@@ -0,0 +1,54 @@
+local helpers = require('test.functional.helpers')
+
+local eq = helpers.eq
+local feed = helpers.feed
+local clear = helpers.clear
+local funcs = helpers.funcs
+local command = helpers.command
+local curbufmeths = helpers.curbufmeths
+
+before_each(function()
+  clear()
+  curbufmeths.set_lines(0, 1, true, { 'foo', 'bar', 'baz' })
+end)
+
+local buffer_contents = function()
+  return curbufmeths.get_lines(0, -1, false)
+end
+
+local cmdtest = function(cmd, prep, ret1)
+  describe(':' .. cmd, function()
+    it(cmd .. 's' .. prep .. ' the current line by default', function()
+      command(cmd .. '\nabc\ndef\n')
+      eq(ret1, buffer_contents())
+    end)
+    -- Used to crash because this invokes history processing which uses 
+    -- hist_char2type which after fdb68e35e4c729c7ed097d8ade1da29e5b3f4b31 
+    -- crashed.
+    it(cmd .. 's' .. prep .. ' the current line by default when feeding',
+    function()
+      feed(':' .. cmd .. '\nabc\ndef\n.\n')
+      eq(ret1, buffer_contents())
+    end)
+    -- This used to crash since that commit as well.
+    it('opens empty cmdline window', function()
+      local hisline = '" Some comment to be stored in history'
+      feed(':' .. hisline .. '<CR>')
+      feed(':' .. cmd .. '<CR>abc<CR>def<C-f>')
+      eq({ 'def' }, buffer_contents())
+      eq(hisline, funcs.histget(':', -2))
+      eq(cmd, funcs.histget(':'))
+      -- Test that command-line window was launched
+      eq('nofile', curbufmeths.get_option('buftype'))
+      eq('n', funcs.mode(1))
+      feed('<CR>')
+      eq('c', funcs.mode(1))
+      feed('.<CR>')
+      eq('n', funcs.mode(1))
+      eq(ret1, buffer_contents())
+    end)
+  end)
+end
+cmdtest('insert', ' before', { 'abc', 'def', 'foo', 'bar', 'baz' })
+cmdtest('append', ' after', { 'foo', 'abc', 'def', 'bar', 'baz' })
+cmdtest('change', '', { 'abc', 'def', 'bar', 'baz' })