Minor optimization to 'svn log -g'.

* subversion/libsvn_repos/log.c
  (fs_mergeinfo_changed): Defer hash key access and pool cleanup until
                          we may use them.  Also, handle modified nodes
                          first since they are the majority.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586924 13f79535-47bb-0310-9956-ffa450edef68