Factor out common code.  No functional change.

* tools/dist/release.py
  (create_tag): Introduce a new local helper, use it to simplify two similar
    blocks of code and reduce the number of local variables.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1485563 13f79535-47bb-0310-9956-ffa450edef68