Make magit-ediff-restore work when there is several diff open at once.

We use local-variable set in the ediff control buffer by the
startup-hook, and the ediff-cleanup-hook in place of ediff-quit-hook.
Added one forgotten startup-hook in magit-ediff*
When merging conflict, use ediff handling of merged file
When solving conflict, ask ediff to look only at conflicting part of files