On the verify-at-commit branch, add a backend-dependent implementation (FSFS):

This required devising a way for an FSAP backend to call svn_fs_open().

* subversion/libsvn_fs/fs-loader.h
  (fs_library_vtable_t.set_svn_fs_open): New API.

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_create, svn_fs_open, svn_fs_create_berkeley, svn_fs_open_berkeley):
    Invoke it from all public APIs that create/populate an svn_fs_t.

* subversion/libsvn_fs_base/fs.c
  (base_set_svn_fs_open, library_vtable): Add no-op implementation.

* subversion/libsvn_fs_fs/fs.c
  (fs_set_svn_fs_open, library_vtable): Add non-no-op implementation, using..

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_data_t.svn_fs_open_): .. this new struct member.
  
* subversion/libsvn_fs_fs/fs_fs.c
  (verify_as_revision_before_current_plus_plus): New.
  (commit_body): Call it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/verify-at-commit@1462062 13f79535-47bb-0310-9956-ffa450edef68