Fix the hinting function in svn_sort__array_lookup.

On trunk, we use it in a way that does not expose the breakage
caused by only comparing one side of the insertion position.
Take the opportunity to write clearer and less restrictive code
by special-casing insertion at the array boundaries.

* subversion/libsvn_subr/sorts.c
  (svn_sort__array_lookup): Rewrite the HINT evaluation code path.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1621375 13f79535-47bb-0310-9956-ffa450edef68