Avoid IllegalArgumentException when auto-diving in root paths

The checkArgument() call in GitilesView.getBreadcrumbs(List<Boolean>)
is intended to avoid programmer error due to the subtle relationship
between hasSingleTree and the view's path. In fact, the check was
incorrectly failing when browsing root paths like "/+/master/". Change
the check to fix that case.

Change-Id: I1a1f8b57d5d16b45ec4da32afaf2009936486787