shell: Ensure silent bang mappings won't cut output

Setting `msg_didout` after `call_shell` is enough as it will cause
`hit_return_msg()` to print on next line.

Close #3269

src/nvim/ex_cmds.c

@@ -1246,8 +1246,9 @@ do_shell (
   // 1" command to the terminal.
   ui_cursor_goto(msg_row, msg_col);
   (void)call_shell(cmd, flags, NULL);
-  did_check_timestamps = FALSE;
-  need_check_timestamps = TRUE;
+  msg_didout = true;
+  did_check_timestamps = false;
+  need_check_timestamps = true;
 
   // put the message cursor at the end of the screen, avoids wait_return()
   // to overwrite the text that the external command showed

test/functional/helpers.lua

@@ -372,5 +372,6 @@ return {
   set_session = set_session,
   write_file = write_file,
   rmdir = rmdir,
+  mkdir = lfs.mkdir,
   exc_exec = exc_exec,
 }

test/functional/shell/bang_filter_spec.lua

@@ -0,0 +1,49 @@
+-- Specs for bang/filter commands
+
+local helpers = require('test.functional.helpers')
+local feed, execute, clear = helpers.feed, helpers.execute, helpers.clear
+local mkdir, write_file, rmdir = helpers.mkdir, helpers.write_file, helpers.rmdir
+
+local Screen = require('test.functional.ui.screen')
+
+
+describe('issues', function()
+  local screen
+
+  before_each(function()
+    clear()
+    rmdir('bang_filter_spec')
+    mkdir('bang_filter_spec')
+    write_file('bang_filter_spec/f1', 'f1')
+    write_file('bang_filter_spec/f2', 'f2')
+    write_file('bang_filter_spec/f3', 'f3')
+    screen = Screen.new()
+    screen:attach()
+  end)
+
+  after_each(function()
+    rmdir('bang_filter_spec')
+  end)
+
+  it('#3269 Last line of shell output is not truncated', function()
+    execute([[nnoremap <silent>\l :!ls bang_filter_spec<cr>]])
+    feed([[\l]])
+    screen:expect([[
+      ~                                                    |
+      ~                                                    |
+      ~                                                    |
+      ~                                                    |
+      ~                                                    |
+      ~                                                    |
+      ~                                                    |
+      ~                                                    |
+      :!ls bang_filter_spec                                |
+                                                           |
+      f1                                                   |
+      f2                                                   |
+      f3                                                   |
+      Press ENTER or type command to continue^              |
+    ]])
+  end)
+
+end)