With this change, in merge changes, patch ranges can diff against a
specific parent (as indicated by a negative parent index) similarly to
the GWT UI.
- Parent options appear in patch range selectors when available.
- The router no longer redirects parent indexed patch ranges now that
they are supported.
- The RevisionInfo class is added to house revision related functions in
a form that's easily passed between components.
- On merge changes the default patch range base is labeled as
"AutoMerge" rather than "Base".
Feature: Issue 4760
Change-Id: I221ba97e28be52f225f7d90f5f8c5a0f17ddb8c2
a744e2303f