`magit-refresh-wrapper': only refresh status buffer if it has company

Among other things, commit 03d99ca0 changed `magit-refresh-wrapper' to
refresh the status buffer even if `magit-refresh-needing-buffers' is nil.

Given that (a) this causes point to jump to the beginning of the status
buffer every time we open a file from it, and (b) the change wasn't
motivated properly, revert it.

Fixes #687.  Replaces #906.

Signed-off-by: Pieter Praet <pieter@praet.org>