On the new fsfs-format7 branch: allow for txn root nodes to be
created without specifying dummy node and copy IDs.

This is part of the "numerical IDs" patch series.

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_txn_create_root): declare new internal API

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_txn_create_root): implement it

* subversion/libsvn_fs_fs/transaction.c
  (svn_fs_fs__get_txn,
   commit_body): call the new more specific API

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1454229 13f79535-47bb-0310-9956-ffa450edef68