Fix getRevisionChangeset API call making etherpad crash

Using `getRevisionChangeset` makes etherpad crash.

Here's my log:
    [2013-11-07 22:07:05.331] [INFO] API - REQUEST, v1.2.8:getRevisionChangeset, {"apikey":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx","padID":"test"}
    [2013-11-07 22:07:05.371] [INFO] API - RESPONSE, getRevisionChangeset, 500
    [2013-11-07 22:07:05.413] [ERROR] console - TypeError: Property 'callback' of object #<Object> is not a function
        at /var/www/etherpad/src/node_modules/ueberDB/CloneAndAtomicLayer.js:139:17
        at exports.database.getSub (/var/www/etherpad/src/node_modules/ueberDB/CacheAndBufferLayer.js:378:7)
        at exports.database.get (/var/www/etherpad/src/node_modules/ueberDB/CacheAndBufferLayer.js:157:5)
        at exports.database.getSub (/var/www/etherpad/src/node_modules/ueberDB/CacheAndBufferLayer.js:350:8)
        at doOperation [as operatorFunction] (/var/www/etherpad/src/node_modules/ueberDB/CloneAndAtomicLayer.js:133:18)
        at exports.channels.emit (/var/www/etherpad/src/node_modules/ueberDB/node_modules/channels/channels.js:38:11)
        at exports.database.getSub (/var/www/etherpad/src/node_modules/ueberDB/CloneAndAtomicLayer.js:77:17)
        at Pad.getRevisionChangeset (/var/www/etherpad/src/node/db/Pad.js:123:6)
        at exports.getRevisionChangeset (/var/www/etherpad/src/node/db/API.js:190:40)
        at Object.exports.getPad (/var/www/etherpad/src/node/db/PadManager.js:162:5)

Here's a patch making it work correctly.