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

This is similar to the situation described about svn_hash_sets() in
r1507155.  However, we can't use the fix used there because svn_hash_gets()
has a return value.  This essentially roles back the opportunity for the
compiler to optimize the strlen() call when the key is a literal string.  It
also puts svn_hash_gets() back to the same implementation it had in r1443387.

* subversion/include/svn_hash.h
  (svn_hash_gets): Use APR_HASH_KEY_STRING as length.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507366 13f79535-47bb-0310-9956-ffa450edef68