On the fsfs-improvements branch:  Rename the uniquifier.txn_id member
in representation_t to avoid confusion with the txn_id member.
No functional change.

* subversion/libsvn_fs_fs/fs.h
  (representation_t): rename uniquifier.txn_id to uniquifier.noderev_txn_id;
                      extend commentary

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__parse_representation,
   svn_fs_fs__unparse_representation): update caller

* subversion/libsvn_fs_fs/transaction.c
  (set_uniquifier): update caller

Suggested by: danielsh

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-improvements@1510134 13f79535-47bb-0310-9956-ffa450edef68