Add time stamp to commit.
Commit time stamp represents UTC time instead of local time.

In order to avoid potential conflicts with time zones, Commit#time
represents the commit time at coordinated universal time (UTC). I highly
doubt time zone conflicts would ever be an issue, as the time zone is
stored in Commit#time. Assuredly, any time comparisons would consider
the time zone.
Made commit.rb a bit more readable.