ensure that internal methods can't pretend to be git commands

Hub::Context methods were public, now they are private. Ensures that
`Hub::Commands.respond_to?` doesn't return true for any of its methods.

Closes mislav#62

lib/hub/context.rb

@@ -2,6 +2,8 @@ module Hub
   # Provides methods for inspecting the environment, such as GitHub user/token
   # settings, repository info, and similar.
   module Context
+    private
+
     # Caches output when shelling out to git
     GIT_CONFIG = Hash.new do |cache, cmd|
       result = %x{git #{cmd}}.chomp

test/hub_test.rb

@@ -685,6 +685,10 @@ def test_no_browser
     end
   end
 
+  def test_context_method_doesnt_hijack_git_command
+    assert_command 'remotes', 'git remotes'
+  end
+
   protected
 
     def stub_github_user(name)