Merge branch 'dp/filter-branch-prune-empty'

Merge branch 'dp/filter-branch-prune-empty'

"git filter-branch --prune-empty" drops a single-parent commit that
becomes a no-op, but did not drop a root commit whose tree is empty.

* dp/filter-branch-prune-empty:
  p7000: add test for filter-branch with --prune-empty
  filter-branch: fix --prune-empty on parentless commits
  t7003: ensure --prune-empty removes entire branch when applicable
  t7003: ensure --prune-empty can prune root commit