allow displaying the log-margin in the status buffers

Unlike in log buffers the log-margin is always hidden initially (because
it wastes to much space), there is no option to change that, and there
also is no key binding for `magit-log-toggle-margin' (because it should
be the same as in the log buffer, but there it is `h', which isn't good,
especially not in the status buffer, where many beginners will press `h'
expecting help.