The motivation for this CL is:
When a project is turned on to use GitCheckout with try job verifiers, then the CQ crashes because of line 64 in http://src.chromium.org/viewvc/chrome/trunk/tools/commit-queue/pending_manager.py

This is because revision can only be one of (None, int, unicode). This crash only happens when try jobs are used (which is why I did not see it before).
This CL changes the return type to be unicode instead of string.

Review URL: https://chromiumcodereview.appspot.com/25686005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@226743 0039d316-1c4b-4281-b951-d872f2087c98