Add -v/--verbose switch to bin/commit-stats

When supplied, this switch makes bin/commit-stats not only print
aggregate information, but also detailed counts (additions, deletions)
and commit subject lines for each commit.

Rugged proves to be a little flakey; we can reliably observe it
segfaulting after flushing the output buffer. We will probably swap it
out for something else soon.

Change-Id: If59a48cb69846cd4b8ef7be41e656dfce64cea58
Reviewed-on: https://gerrit.causes.com/24869
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>