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

"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