Set an output parameter.  Also refactor code.

The output parameter is NEW_REV.  It will now be set before anything else
happens in this function, regardless of compile-time options.

The refactoring rejiggers the various #ifdef'd blocks, but should have no
functional change.

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_commit_txn): As above.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461743 13f79535-47bb-0310-9956-ffa450edef68