Revert the remainder of 1483532 and replace it with a much safer /
simpler solution.

Suggested by: julianfoad

* subversion/include/svn_hash.h
  (svn_hash_gets,
   svn_hash_sets): calculate key length statically when possible
  (svn_hash_gets_fixed_key,
   svn_hash_sets_fixed_key): drop

* subversion/libsvn_fs_fs/tree.c
  (crawl_directory_dag_for_mergeinfo): switch back to svn_hash_gets /
                                       svn_hash_sets

* subversion/libsvn_ra_svn/client.c
  (ra_svn_log): same

* subversion/libsvn_repos/log.c
  (fill_log_entry): same

* subversion/libsvn_repos/rev_hunt.c
  (svn_repos_get_committed_info): same

* subversion/libsvn_subr/compat.c
  (svn_compat_log_revprops_clear, svn_compat_log_revprops_out): same


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1484181 13f79535-47bb-0310-9956-ffa450edef68