Diff improvements

This PR adds a new option to the diff tree command, to show a summary of changes by trees

It also adds a new command to export the diffs between two commits as a new shapefile (this can be expanded in the future to support the other supported export formats)

All this together makes it easier to handle the repository from a geogit client, specially in the case of diffs involving a large number of changes, since a feature-based diff might not be useful in that case.