patch 8.1.0640: get E14 while typing command :tab with 'incsearch' set

Problem:    Get E14 while typing command :tab with 'incsearch' set.
Solution:   Do not give an error when looking for the command. (Yasuhiro
            Higashi)
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 3c83dd1..86df794 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -783,6 +783,20 @@
   bwipe!
 endfunc
 
+func Test_incsearch_cmdline_modifier()
+  if !exists('+incsearch')
+    return
+  endif
+  call test_override("char_avail", 1)
+  new
+  call setline(1, ['foo'])
+  set incsearch
+  " Test that error E14 does not occur in parsing command modifier.
+  call feedkeys("V:tab", 'tx')
+
+  call Incsearch_cleanup()
+endfunc
+
 func Test_incsearch_scrolling()
   if !CanRunVimInTerminal()
     return