Allow diff generation starting at a base commit

Review Request #7322 — Created May 20, 2015 and submitted — Latest diff uploaded

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dvcs
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Reviewers

The diffviewer can now generate a commit starting at a certain commit.
This allows the diffs of individual commits in a commit history to be
viewed, as well as spans of commits.

This change also updates the WebAPI's DiffContextResource so that the
commit selection UI can request ranges of commits instead of the entire
history.

Ran unit tests.