Improve memory usage while fetching changed paths data from a transaction
by eliminating a full in-memory copy of changed path data.

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__read_changes_incrementally): New FSFS-internal API which
   invokes a callback for each changed path in a transaction.

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__read_changes_incrementally): Declare.

* subversion/libsvn_fs_fs/transaction.c
  (svn_fs_fs__txn_changes_fetch): Use svn_fs_fs__read_changes_incrementally(),
   instead of reading a changes_t list into memory and looping over that list.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604188 13f79535-47bb-0310-9956-ffa450edef68