More 'svn log -g' tuning.

We need to detect path renames (including parent path renames),
which is relatively expensive.  Therefore, check first if there
have been any renames in the current revision.  If there weren't
any, skip the rename check.

Since we add a full iteration loop for the above, let's move
the check for potenial mergeinfo changes up there as well
(quick early exit).

* subversion/libsvn_repos/log.c
  (fs_mergeinfo_changed): See above.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586925 13f79535-47bb-0310-9956-ffa450edef68