Redirect to normalized versions of parent revision expressions

Repeatedly clicking the "parent" link in a commit page can result in
amusing but unhelpful URLs like "/+/master^^^^^^^^^^^^". Avoid such
URLs by detecting when a revision is a parent expression, and sending
a 302 redirect to the absolute revision.

Change-Id: I8aec460a99eba581052e0319069eacd8f3e2e743
5 files changed