Add SVN_FS_CONFIG_FSFS_SHARD_SIZE fs config option to create FSFS
repositories with custom shard size. Use it in FSFS tests for now.

* subversion/include/svn_fs.h
  (SVN_FS_CONFIG_FSFS_SHARD_SIZE): New.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__create): Handle SVN_FS_CONFIG_FSFS_SHARD_SIZE fs config option.

* subversion/tests/svn_test_fs.h
  (svn_test__create_fs2): New function declaration.

* subversion/tests/svn_test_fs.c
  (create_fs): Add OVERLAY_FS_CONFIG argument to overlay standard test
   fs config values.
  (svn_test__create_fs2): Revv from svn_test__create_fs(). Pass FS_CONFIG to
   create_fs() function.
  (svn_test__create_fs): Call svn_test__create_fs2() with OVERLAY_FS_CONFIG
   set to NULL.

* subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
  (write_format): Remove.
  (create_packed_filesystem): Use svn_test__create_fs2() with a custom
   SVN_FS_CONFIG_FSFS_SHARD_SIZE fs config value instead of rewriting 
   format file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1624512 13f79535-47bb-0310-9956-ffa450edef68