magit.el: introduce magit-simple-log for graph-less log

magit-log is only useful when navigating recent history.  To navigate
larger histories, it is imperative to turn off the graph and increase
the default number of entries displayed.  Introduce a magit-simple-log
that does exactly this.  While at it, get contrib/magit-simple-keys.el
to call it instead of magit-log.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>