Revert "fix issue 628"

This reverts commit f0a9ca8b871876ccfbc5cdd8e7eb99f407d2a55b.  It is the
cause for a very serious regression that makes Emacs hang and crash
randomly on log operations.  While the exact nature of the bug is still
unknown, revert this immediately.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Introduce magit-log-simple 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-log-simple
that does exactly this.  Also teach magit-key-mode.el about it, and
switch contrib/magit-simple-keys.el to use it by default.

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