magit-status-mode-map: bind h to magit-log-toggle-margin

This is the same binding as in `magit-log-mode-map'.  We should
probably find a better key (because users would expect "h" to
be bound to a help command) in both maps.  But that can wait
until later.
magit-log-section-args: doc-string cosmetics
prepare the log margin upfront

Previously, when the margin was disabled, we did not create the overlays
that make up the margin to avoid unnecessary work.  That work is not
very involved so we did not save much.  What is involved though is
having to recreate the complete log buffer when later enabling the
margin; and so it is better to do this right away.
only add a pseudo fringe to the log margin in log buffers

Then showing the log margin in a non-log buffer most lines don't have a
margin overlay and so all of the margin for these lines use the default
face.  Therefor we cannot add pseudo fringe to the margins of log lines
or it would look bad.
Merge branch 'n/log-margin' into next