Added WinRM grep command filter

This is needed because isn't available on Windows and Vagrant guest detection attempts to use grep for some OSs.

plugins/communicators/winrm/command_filter.rb

@@ -8,6 +8,7 @@ module VagrantPlugins
         "cat",
         "chmod",
         "chown",
+        "grep",
         "rm",
         "test",
         "uname",

plugins/communicators/winrm/command_filters/grep.rb

@@ -0,0 +1,26 @@
+module VagrantPlugins
+  module CommunicatorWinRM
+    module CommandFilters
+
+      # Converts a *nix 'grep' command to a PowerShell equivalent
+      class Grep
+
+        def filter(command)
+          # grep 'Fedora release [12][67890]' /etc/redhat-release
+          # grep Funtoo /etc/gentoo-release
+          # grep Gentoo /etc/gentoo-release
+
+          # grep is often used to detect the guest type in Vagrant, so don't bother running
+          # to speed up OS detection
+          ''
+        end
+
+        def accept?(command)
+          command.start_with?('grep ')
+        end
+
+      end
+
+    end
+  end
+end

test/unit/plugins/communicators/winrm/command_filter_test.rb

@@ -15,6 +15,10 @@ describe VagrantPlugins::CommunicatorWinRM::CommandFilter, :unit => true do
       expect(subject.filter('uname -s stuff')).to eq('')
     end
 
+    it 'filters out grep commands' do
+      expect(subject.filter("grep 'Fedora release [12][67890]' /etc/redhat-release")).to eq("")
+    end
+
     it 'filters out which commands' do
       expect(subject.filter('which ruby')).to include(
         '[Array](Get-Command ruby -errorAction SilentlyContinue)')