Fix pool usage in mergeinfo elision code; the current code was leading

Fix pool usage in mergeinfo elision code; the current code was leading
to assertion failures in reintegrate, among other things.

Specifically, it was assuming in two places that paths passed to
callbacks would live longer than they actually do.

* subversion/libsvn_client/mergeinfo.c
  (elide_mergeinfo_catalog_open_directory): Copy path into directory
   pool.
  (struct elide_mergeinfo_catalog_cb_baton): Add result_pool member.
  (elide_mergeinfo_catalog_cb): Copy elidable path into result_pool.
  (svn_client__elide_mergeinfo_catalog): Initialize result_pool
   member.

git-svn-id: http://svn.collab.net/repos/svn/trunk@30820 612f8ebc-c883-4be0-9ee0-a4e9ef946e3a