Fix a backward compat in FSFS: when writing repositories formats that
don't support rep sharing, we must not store the uniquifiers.

* subversion/libsvn_fs_fs/transaction.c
  (set_uniquifier): Write uniquifiers only when supported.

Found by: Evgeny Kotkov <evgeny.kotkov{_AT_}visualsvn.com>

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1561426 13f79535-47bb-0310-9956-ffa450edef68