Merge branch 'dev/multiple_repo' into next

* dev/multiple_repo:
  plugin/magit.vim: in show_magit(), always open buffer with :buffer command (fix #41)
  plugin/magit.vim,autoload/git.vim: yet another rework of magit#show_magit() (fix #41)
  plugin/magit.vim,autoload/{git,sign}.vim: magit can handle multiple git repository within one vim session (fix #44,#34,#29)