Support for release based reporting

A feature to allow a user to specify 2 refspecs, either branches or tags. The statistics should walk through git to the locate the common parent of the 2 branches or tags, and then use that as the initial commit to count for contributions reporting in the reporting branch.

For example if if a project has a 1.0 release and a 2.0 release, we would be interested in identifying and getting statistics on all of the contributions to the 2.0 release.