Fix a theoretical bug in the ra-serf get-mergeinfo implementation.
Fix a theoretical bug in the ra-serf get-mergeinfo implementation.

* subversion/libsvn_ra_serf/mergeinfo.c
  (svn_ra_serf__get_mergeinfo): Fix this to use the new
    svn_ra_serf__get_baseline_info() helper, which accurately handle the
    case where the peg revision is SVN_INVALID_REVNUM.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion