FS: Allow the 'verify' API to run against transaction roots.

* subversion/include/svn_fs.h
  (svn_fs_verify_rev): Change signature.

* subversion/libsvn_repos/dump.c
  (svn_repos_verify_fs2): Update caller.

* subversion/libsvn_fs/fs-loader.c
  (svn_repos_verify_fs2): Propagate signature change.

* subversion/libsvn_fs/fs-loader.h
  (fs_vtable_t.verify_rev): Update signature.

* subversion/libsvn_fs_base/fs.c
  (base_bdb_verify_rev): Track signature change.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__verify_rev): Track signature change, adjust code accordingly.

* subversion/libsvn_fs_fs/fs_fs.h
  (svn_fs_fs__verify_rev): Track signature change.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461335 13f79535-47bb-0310-9956-ffa450edef68