In case of in-repo authz data, we usually already have an open repository.
So, add that as an optional parameter to to config_pool getter to save the
expensive repo_open call.

* subversion/include/private/svn_repos_private.h
  (svn_repos__config_pool_get): add optional preferred_repo parameter

* subversion/libsvn_repos/config_pool.c
  (find_repos_config): use repo provided to us if that matches the path
  (svn_repos__config_pool_get): parameter pass-through

* subversion/tests/libsvn_repos/repos-test.c
  (test_config_pool): update API caller

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1538120 13f79535-47bb-0310-9956-ffa450edef68