Implement svn_repos_fs_type() for easy access to repository filesystem type.

* subversion/include/svn_repos.h
* subversion/libsvn_repos/repos.c
  (svn_repos_fs_type): New.

* subversion/tests/libsvn_repos/repos-test.c
  (test_repos_fs_type): New test for svn_repos_fs_type() function.
  (test_funcs): Add test_repos_fs_type.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1538585 13f79535-47bb-0310-9956-ffa450edef68