Remove file/path completion for Grepper command

As far as I can tell, this does not make sense for the Grepper command.
The command opens the prompt, from which path completion is performed.

I suspect the right thing to do here is complete with all flags, but it
seems sensible to leave that for another commit to make changes clear.

plugin/grepper.vim

@@ -228,7 +228,7 @@ function! grepper#complete(lead, line, _pos) abort
     return filter(map(sort(copy(g:grepper.tools)), 'v:val." "'),
           \ 'empty(a:lead) || v:val[:strlen(a:lead)-1] ==# a:lead')
   else
-    return grepper#complete_files(a:lead, 0, 0)
+    return []
   endif
 endfunction
 

test/feature/completion.vader

@@ -37,6 +37,9 @@ Execute (command: flags, -tool options):
   Assert len(grepper#complete('', 'Grepper -tool ', v:null)) > 1
   AssertEqual count(grepper#complete('', 'Grepper -tool ', v:null), 'grep '), 1
 
+Execute (command: flags, default, no completion):
+  Assert len(grepper#complete('', '', v:null)) == 0
+
 Execute (prompt: path, tilde expansion):
   AssertEqual grepper#complete_files('x ~', v:null, v:null), ['x '.$HOME]