Fix ra_svn/svnserve implementation of get-mergeinfo to use the

Fix ra_svn/svnserve implementation of get-mergeinfo to use the
documented protocol, instead of adding some extra layers of nesting
and then accidentally discarding all but the first key/value pair
returned.  (ie, make svn_ra_get_mergeinfo with multiple targets or
with include_descendants work properly over ra_svn.)

* subversion/libsvn_ra_svn/client.c
  (ra_svn_get_mergeinfo): Parse the body of the response (ie, the X
   in:
     ( success ( X ) )
   ) as a list, not a list inside yet another list.  And it's not
   optional.

* subversion/svnserve/serve.c
  (get_mergeinfo): Don't wrap each key/value pair in more parentheses
   than svn_ra_svn_write_tuple already gives you.

git-svn-id: http://svn.collab.net/repos/svn/trunk@29489 612f8ebc-c883-4be0-9ee0-a4e9ef946e3a