Support base commit IDs for diff validation.

Review Request #7015 — Created March 6, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
a5e7450...

Reviewers

The diff validation resource didn't have any support for base commit
IDs, which broke posting diffs against Bitbucket repositories. It now
accepts a base_commit_id field in the API, which will be passed to
DiffSetManager.create_from_upload.

There's also a new capability, so that RBTools can determine whether to
validate diffs when it has a base_commit_id value.

Unit tests pass.