Switch svn_fs_fs__read_rep_header() to the dual-pool idiom.

* subversion/libsvn_fs_fs/cached_data.c
  (create_rep_state_body, read_rep_header): Pass a scratch pool.

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

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

* subversion/libsvn_fs_fs/pack.c
  (copy_rep_to_temp): Pass standard pool as scratch pool.

* subversion/libsvn_fs_fs/recovery.c
  (recover_find_max_ids): Pass standard pool as scratch pool.

* subversion/libsvn_fs_fs/transaction.c
  (upgrade_transaction): Pass an iterpool as scratch pool.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604911 13f79535-47bb-0310-9956-ffa450edef68