patch 8.0.0220: completion of highlight names misses a few values

Problem:    Completion for :match does not show "none" and other missing
            highlight names.
Solution:   Skip over cleared entries before checking the index to be at the
            end.
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index abeb609..279a168 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -25,8 +25,34 @@
   set nowildmenu
 endfunc
 
+func Test_match_completion()
+  if !has('cmdline_compl')
+    return
+  endif
+  hi Aardig ctermfg=green
+  call feedkeys(":match \<Tab>\<Home>\"\<CR>", 'xt')
+  call assert_equal('"match Aardig', getreg(':'))
+  call feedkeys(":match \<S-Tab>\<Home>\"\<CR>", 'xt')
+  call assert_equal('"match none', getreg(':'))
+endfunc
+
+func Test_highlight_completion()
+  if !has('cmdline_compl')
+    return
+  endif
+  hi Aardig ctermfg=green
+  call feedkeys(":hi \<Tab>\<Home>\"\<CR>", 'xt')
+  call assert_equal('"hi Aardig', getreg(':'))
+  call feedkeys(":hi li\<S-Tab>\<Home>\"\<CR>", 'xt')
+  call assert_equal('"hi link', getreg(':'))
+  call feedkeys(":hi d\<S-Tab>\<Home>\"\<CR>", 'xt')
+  call assert_equal('"hi default', getreg(':'))
+  call feedkeys(":hi c\<S-Tab>\<Home>\"\<CR>", 'xt')
+  call assert_equal('"hi clear', getreg(':'))
+endfunc
+
 func Test_expr_completion()
-  if !(has('cmdline_compl') && has('eval'))
+  if !has('cmdline_compl')
     return
   endif
   for cmd in [