os/shell: do_os_system(): Always show last chunk.

This ameliorates use-cases like:
    :!cat foo.txt
    :make
where the user is interested in the last few lines of output.

Try these shell-based ex-commands before/after this commit:
    :grep -r '' *
    :make
    :!yes
    :!grep -r '' *
    :!git grep ''
    :!cat foo
    :!echo foo
    :!while true; do date; done
    :!for i in `seq 1 20000`; do echo XXXXXXXXXX $i; done

In all cases the last few lines of the command should always be shown,
regardless of where throttling was triggered.

test/functional/ui/output_spec.lua

@@ -1,6 +1,5 @@
 local session = require('test.functional.helpers')(after_each)
 local child_session = require('test.functional.terminal.helpers')
-local Screen = require('test.functional.ui.screen')
 
 if session.pending_win32(pending) then return end
 
@@ -41,10 +40,24 @@ describe("shell command :!", function()
     ]])
   end)
 
-  it("throttles shell-command output greater than ~20KB", function()
+  it("throttles shell-command output greater than ~10KB", function()
+    screen.timeout = 20000  -- Avoid false failure on slow systems.
     child_session.feed_data(
-      ":!for i in $(seq 2 3000); do echo XXXXXXXXXX; done\n")
-    -- If a line with only a dot "." appears, then throttling was triggered.
+      ":!for i in $(seq 2 3000); do echo XXXXXXXXXX $i; done\n")
+
+    -- If we observe any line starting with a dot, then throttling occurred.
     screen:expect("\n.", nil, nil, nil, true)
+
+    -- Final chunk of output should always be displayed, never skipped.
+    -- (Throttling is non-deterministic, this test is merely a sanity check.)
+    screen:expect([[
+      XXXXXXXXXX 2996                                   |
+      XXXXXXXXXX 2997                                   |
+      XXXXXXXXXX 2998                                   |
+      XXXXXXXXXX 2999                                   |
+      XXXXXXXXXX 3000                                   |
+      {10:Press ENTER or type command to continue}{1: }          |
+      {3:-- TERMINAL --}                                    |
+    ]])
   end)
 end)