Extend API to allow changesets to be queried by ids

Make changesets queryable by specifying a list of ids, for example:

    /api/0.6/changesets?changesets=1,2,3

This condition may be combined with others in the normal way.

test/functional/changeset_controller_test.rb

@@ -1543,6 +1543,13 @@ EOF
     get :query, :closed => 'true', :user => users(:public_user).id
     assert_response :success, "can't get changesets by closed-ness and user"
     assert_changesets [7]
+
+    get :query, :changesets => '1,2,3'
+    assert_response :success, "can't get changesets by id (as comma-separated string)"
+    assert_changesets [1,2,3]
+
+    get :query, :changesets => ''
+    assert_response :bad_request, "should be a bad request since changesets is empty"
   end
 
   ##