When processing APR hashes, reuse the key length information where possible.

* subversion/libsvn_repos/log.c
  (detect_changed): get & reuse key length
  (send_log): same; also reduce number of temp. pools 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483572 13f79535-47bb-0310-9956-ffa450edef68