On the fsfs-format7 branch: Change the way the proto-l2p-index is being
created.  Instead of implicitly assigning logical item indexes, specify
then explicitly.

Also, provide a function to convert an item_index into a file offset.
It is suitable for direct as well as logical addressing.

* subversion/libsvn_fs_fs/index.h
  (svn_fs_fs__l2p_proto_index_add_offset): renamed to ...
  (svn_fs_fs__l2p_proto_index_add_entry): ... this; add item_index parameter
  (svn_fs_fs__item_offset): declare new internal API function

* subversion/libsvn_fs_fs/index.c
  (l2_proto_index_entry_t): new internal data structure
  (write_number_to_proto_index): renamed to ...
  (write_entry_to_proto_index): ... this; write pairs instead of ints
  (svn_fs_fs__l2p_proto_index_add_revision,
   svn_fs_fs__l2p_proto_index_add_entry,
   svn_fs_fs__l2p_index_create): update
  (svn_fs_fs__l2p_index_lookup): offsets are now stored as offset+1
  (svn_fs_fs__p2l_index_lookup): implement new internal API function

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1432701 13f79535-47bb-0310-9956-ffa450edef68