Resolve several easy to fix compiler warnings.

* subversion/libsvn_fs_fs/lock.c
  (add_to_digest): Use unsigned int type to match apr_hash_count() signature.

* subversion/libsvn_fs_fs/low_level.c
  (read_header_block): Use apr_size_t for indexes instead of apr_ssize_t and
   refactor code to avoid subtraction.

* subversion/libsvn_fs_fs/temp_serializer.c
  (serialize_dir): Use int type for index to match apr_array_header->nelts 
   type.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1626246 13f79535-47bb-0310-9956-ffa450edef68