Added support for pruning the graph.

This release introduced the --choose-branch and --choose-tag options.

They were provided so that if you want to limit the analysis to a small set of branches or tags.

These options prune the graph so that only parents of commits with the choose branch
or tag ids are included in the graph. This gives you more detail controlled that the git options
allowed in the --range command. It is very useful for determining where branches occurred.