magit-merge-popup: improve popup and its actions

* Add new variants `magit-merge-editmsg' (which is closest to the old
  `magit-merge' but not quite the same), `magit-merge-nocommit' and a
  new `magit-merge' (which creates the merge commit right away provided
  there are no conflicts).

* Turn `magit-merge-popup' into a "sequence popup", that is if a merge
  is already in progress offer different actions: `magit-merge-abort'
  and now also `magit-commit'.
Merge branch 'n/merge-popup-1' into next