vcs_PullRequest: Add Source and Target Branch

[BrentSouza]

Pull/Merge Requests generally have information about the head/source and base/target branches of interest, but the current data model for vcs_PullRequest does not capture this information. It would be useful to have source/target branch references as part of vcs_PullRequest to derive workflow information.

[csailer]

Adding image of unit-test results