patch 8.2.0925: getcompletion() does not return command line arguments

Problem:    Getcompletion() does not return command line arguments.
Solution:   Add the "cmdline" option. (Shougo, closes #1140)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 81350aa..1d3f275 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -355,6 +355,20 @@
     call assert_equal(['Testing'], l)
   endif
 
+  " Command line completion tests
+  let l = getcompletion('cd ', 'cmdline')
+  call assert_true(index(l, 'samples/') >= 0)
+  let l = getcompletion('cd NoMatch', 'cmdline')
+  call assert_equal([], l)
+  let l = getcompletion('let v:n', 'cmdline')
+  call assert_true(index(l, 'v:null') >= 0)
+  let l = getcompletion('let v:notexists', 'cmdline')
+  call assert_equal([], l)
+  let l = getcompletion('call tag', 'cmdline')
+  call assert_true(index(l, 'taglist(') >= 0)
+  let l = getcompletion('call paint', 'cmdline')
+  call assert_equal([], l)
+
   " For others test if the name is recognized.
   let names = ['buffer', 'environment', 'file_in_path', 'mapping', 'tag', 'tag_listfiles', 'user']
   if has('cmdline_hist')
@@ -379,7 +393,7 @@
   set tags&
 
   call assert_fails('call getcompletion("", "burp")', 'E475:')
-  call assert_fails('call getcompletion("abc", [])', 'E474:')
+  call assert_fails('call getcompletion("abc", [])', 'E475:')
 endfunc
 
 func Test_shellcmd_completion()