Add svn_fs_commit_txn2 regression test.

* subversion/include/svn_fs.h
  (svn_fs_commit_txn2): Document parameter.

* subversion/tests/libsvn_fs/fs-test.c
  (commit_timestamp): New test.
  (test_funcs): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1546006 13f79535-47bb-0310-9956-ffa450edef68