org-magit.el: Fix org-magit-open on commit link

* org-magit.el (org-magit-open): magit-show-commit shows the buffer but
  doesn't return it.

magit-show-commit shows the buffer for the given commit but doesn't
return the buffer itself. Instead it returns a list of magit
structures (such as magit-section).
Merge pull request #2 from daimrod/fix-org-magit-open-commit-link

org-magit.el: Fix org-magit-open on commit link