refactor: hide stderr from Neovim

This implements idea "3" from here:

- wincent/command-t#393 (comment)

> we may also want to swallow `stderr` so it doesn't leak back in any
> visible way (ie. that would mean changing a call to `git ...` into
> `git ... 2> /dev/null` etc). The only catch is that this may make
> trouble-shooting harder (users could edit out or change the redirect,
> I guess).

So, yeah. This is a double-edged sword, but in the next commit I am
planning on dulling at least one of those edges by implementing a
fallback behavior for each of these scanners.

lua/wincent/commandt/private/scanners/find.lua

@@ -19,7 +19,7 @@ find.scanner = function(directory)
   end
   local lib = require('wincent.commandt.private.lib')
   -- TODO: support max depth, dot directory filter etc
-  local command = 'find -L ' .. directory .. ' -type f -print0'
+  local command = 'find -L ' .. directory .. ' -type f -print0 2> /dev/null'
   local scanner = lib.scanner_new_command(command, drop)
   return scanner
 end

lua/wincent/commandt/private/scanners/git.lua

@@ -15,6 +15,7 @@ git.scanner = function(directory, options)
   if directory ~= '' then
     command = command .. ' -- ' .. directory
   end
+  command = command .. ' 2> /dev/null'
   local scanner = lib.scanner_new_command(command)
   return scanner
 end

lua/wincent/commandt/private/scanners/rg.lua

@@ -15,6 +15,7 @@ rg.scanner = function(directory)
   if #directory > 0 then
     command = command .. ' ' .. directory
   end
+  command = command .. ' 2> /dev/null'
   local scanner = lib.scanner_new_command(command, drop)
   return scanner
 end