Add parent diff support to the JSON API and post-review

Review Request #396 — Created May 21, 2008 and submitted — Latest diff uploaded


Review Board SVN (deprecated)


This adds support for uploading parent diffs in the JSON API, and also introduces a --parent flag to post-review for doing parent diffs on supporting repository types (currently only git-svn). It should be pretty easy to add the support to the other types.
Posted some standard diffs to make sure they didn't break. Then posted a diff with a parent diff. Saw that it uploaded as I expected.