patch 8.0.0878: no completion for :mapclear

Problem:    No completion for :mapclear.
Solution:   Add completion (Nobuhiro Takasaki et al. closes #1943)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index c272376..f9499ac 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -222,6 +222,11 @@
   let l = getcompletion('not', 'messages')
   call assert_equal([], l)
 
+  let l = getcompletion('', 'mapclear')
+  call assert_true(index(l, '<buffer>') >= 0)
+  let l = getcompletion('not', 'mapclear')
+  call assert_equal([], l)
+
   if has('cscope')
     let l = getcompletion('', 'cscope')
     let cmds = ['add', 'find', 'help', 'kill', 'reset', 'show']