Fix the second part of issue #3641.
Fix the second part of issue #3641.

Note: this passes tests, but in pre-commit review I suspect the new 'if'
could be improved, so please review.

* subversion/libsvn_repos/replay.c
  (add_subdir):
    Don't call add_subdir() when there is no history to convert
    to a history-less add.  (Thanks, docstrings.)

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion