Minor FSFS refactoring: move a common code sequence to util.*

* subversion/libsvn_fs_fs/util.h
  (svn_fs_fs__packed_base_rev): Declare new internal API.

* subversion/libsvn_fs_fs/util.c
  (svn_fs_fs__packed_base_rev): Implement it with code taken from rev_file.c.

* subversion/libsvn_fs_fs/rev_file.c
  (init_revision_file,
   open_pack_or_rev_file): Call the new function.

* subversion/libsvn_fs_fs/verify.c
  (packed_base_rev): Remove local function.
  (verify_index_consistency): Call the new one.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1623368 13f79535-47bb-0310-9956-ffa450edef68