Make -r invalid@rev a warning and ignore the argument.
Make -r rev use the Nth solution.
Fix a bug in FakeRepos.assertTree().

TEST=updated the smoke test to reflect the new behavior which should be the most compatible with the current buildbot behavior
BUG=chromium-os:3465

Review URL: http://codereview.chromium.org/2430001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@48607 0039d316-1c4b-4281-b951-d872f2087c98