Switch svn_fs_fs__read_changes() to the dual-pool idiom.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_changes, block_read_changes): Pass a scratch_pool.

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__read_changes): Switch to dual-pool. Use scratch pool
   for temporary allocations.

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__read_changes): Tweak declaration and docstring.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604902 13f79535-47bb-0310-9956-ffa450edef68