how to robustly use `git log` and limit the commits by date/time

* show that CommitDate is considered, when using `--after` and `--before`
* show that `--after` means on-or-after and `--before` means on-or-before
* best practices: show how to limit by absolute time (ISO format),
  so that the log command's results are identical for everyone around the
  world
* format dates returned by git log according to your timezone of choice,
  using TZ (tested on GNU/Linux and Windows) and `--date=local`
* explain how `--after=2008-06-01` default to using time when command is
  run; and how offsets from UTC are (in this case) not corrected for
  DST changes.