Introduce path construction utilities for the 'transactions' and
'txn-protorevs' directory in FSFS repositories.

* subversion/libsvn_fs_fs/util.h
  (svn_fs_fs__path_txns_dir,
   svn_fs_fs__path_txn_proto_revs): Declare new private API.

* subversion/libsvn_fs_fs/util.c
  (svn_fs_fs__path_txns_dir): Implement new private API.
  (svn_fs_fs__path_txn_dir): Use the new function.
  (svn_fs_fs__path_txn_proto_revs): Implement new private API.
  (svn_fs_fs__path_txn_proto_rev,
   svn_fs_fs__path_txn_proto_rev_lock): Use the new function.

* subversion/libsvn_fs_fs/fs_fs.c
  (upgrade_body,
   svn_fs_fs__create_file_tree): Use the new path constructors.

* subversion/libsvn_fs_fs/transaction.c
  (create_txn_dir_pre_1_5,
   svn_fs_fs__list_transactions): Same.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1630070 13f79535-47bb-0310-9956-ffa450edef68