The diffviewer can now set the condensed_diff and base_commit_id on

individual diff comments in the database. These attributes are piped

from the RB.DiffViewerPageView all the way to the RB.DiffComment

model.
Some methods have been added to the RB.DiffCommitIndexView for

determining the currently selected range, if an inter-commit diff is

being displayed, and for determining the "distance" between two commits

(assuming the history is linear).

Made comments on diff commits. Checked the outgoing requests and

incoming responses and verified they were correct.