magit-goto-section: don't start showing details in long log

When moving between commit in the log buffer the commit buffer is
automatically updated to show all information about commit at point.

If the commit buffer is not already displayed in the current frame then
it is automatically displayed in another window.

Previously this was done for both "oneline" and "long" logs.  Now it is
only done for "oneline" logs, because otherwise there would be little
reason to use the "long" log - since the details are also displayed in
the commit buffer using "long" log just wastes space.  Of course it is
still possible to explicitly display the commit buffer, and once it has
been created it is also updated.