We collect and accumulate data based on the order of the output of "git
log", which is not necessarily ordered by timestamp. Adding --date-order
should improve the situation, but isn't sufficient at least on git.git's
repository.

In addition, when encountering a date that is prior to the last one
encountered, we change it to be the last encountered date. A better
solution would be to actually order the lines before starting to count.

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>