Sometimes rietveld returns a datetime without the milliseconds part,
presumably when it is 0. This patch fixes the parsing of those cases.

BUG=chromium:323615
TEST=Ran "my_activity.py --week_of 11/18/13 --user jsbell" which returns "2013-11-18 20:42:30" for a bug.

Review URL: https://codereview.chromium.org/204013009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@258189 0039d316-1c4b-4281-b951-d872f2087c98