Auto-dive into trees that contain only a single subtree

When walking into a tree in PathServlet, keep track of which path
components only contained a single subtree entry, and pass this
information along to GitilesView.getBreadcrumbs(). This creates URLs
with a special sentinel ?stay to disable auto-diving.

Bug: https://code.google.com/p/gitiles/issues/detail?id=8
Change-Id: I03bfb8a6e1974cc1c4b5d836c698311705569ff4