Add note about git log options

Address #745.

@drj11 and @jiffyclub Could you review this?

> `--oneline` is probably the most useful option at this point, maybe start with just that?

I don't want to expand this note too much. I hope that instructors already had
taught `git help` at this point so students can `git help log` and read what
`--oneline`, `--graph`, ... are for.

If more people believe that the write version of the lesson should have this
incremental mention of options I'm OK in change the PR.