Fix possible duplication of work when using svn_hash_sets() macro.

svn_hash_sets() is a macro that uses the key argument twice, so the expression
used in the key argument will be evaluated twice.  We often put calls to
apr_pstrdup() and other functions in that position, which causes unnecessary
extra work and memory usage.

* subversion/include/svn_hash.h
  (svn_hash_sets): Use a variable to avoid executing key expression twice.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507155 13f79535-47bb-0310-9956-ffa450edef68