On the fsfs-format7 branch:  Add a function that tests whether a given ID
is an ID with a transaction.  That eliminates the need for users to get
and check the TXN_ID member themselves, i.e. this reduces the dependencies
on the internal representation of said TXN_ID.

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

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_is_txn): declare new private API

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_is_txn): implement

* subversion/libsvn_fs_fs/cached_data.c
  (get_node_revision_body,
   locate_dir_cache): use the new API

* subversion/libsvn_fs_fs/dag.c
  (svn_fs_fs__dag_check_mutable): ditto

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__write_changes): same here

* subversion/libsvn_fs_fs/transaction.c
  (create_new_txn_noderev_from_rev,
   write_final_rev): and here

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1454582 13f79535-47bb-0310-9956-ffa450edef68