Add an option in fsfs.conf to select the ZLIB compression level that we
apply with txdelta to new revisions.

For near-incompressible data like office documents, artwork etc, this fully
backward compatible option massively speeds up commits and slightly speeds
up exports / checkouts.  Deltification can be controlled independently.

* subversion/libsvn_fs_fs/fs.h
  (CONFIG_OPTION_COMPRESSION_LEVEL): Define the new config option name.
  (fs_fs_data_t): Add config option value.

* subversion/libsvn_fs_fs/fs_fs.c
  (read_config): Extend parser to read the new option.
  (write_config): Update config file template.

* subversion/libsvn_fs_fs/transaction.c
  (rep_write_get_baton,
   write_container_delta_rep): Use the new setting for txdelta compression.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1559748 13f79535-47bb-0310-9956-ffa450edef68