properly terminate created subprocesses

This fixes a memory / ressource leak that manifests when computing
stats over big sets of repositories. It was eating more than 8G of
memory for ~15 git repositories.

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>
implement a way to limit the statistics to commits after a start date

This is really useful when computing statistics over a set of
repositories, where some repositories are much older than other.

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>
Document start_date option on the manpage.