Merge branch 'jk/verify-sig-merge-into-void'

Merge branch 'jk/verify-sig-merge-into-void'

"git merge" and "git pull" that merges into an unborn branch used
to completely ignore "--verify-signatures", which has been
corrected.

* jk/verify-sig-merge-into-void:
  pull: handle --verify-signatures for unborn branch
  merge: handle --verify-signatures for unborn branch
  merge: extract verify_merge_signature() helper