Various fixes around "git am" that applies a patch to a history
that is not there yet.

* pt/am-abort-fix:
  am --abort: keep unrelated commits on unborn branch
  am --abort: support aborting to unborn branch
  am --abort: revert changes introduced by failed 3way merge
  am --skip: support skipping while on unborn branch
  am -3: support 3way merge on unborn branch
  am --skip: revert changes introduced by failed 3way merge