On the fsfs-format7 branch: minor code improvement

* subversion/libsvn_fs_fs/transaction.c
  (choose_delta_base): factor out common code

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1453766 13f79535-47bb-0310-9956-ffa450edef68