Follow up to r1568871: Fix a bug in ra_serf's implementation of svn_ra_get_log
where the wrong peg revision would be chosen if the start of the range is
SVN_INVALID_REVNUM, but the end of the range is an actual revision number.

* subversion/libsvn_ra_serf/log.c (svn_ra_serf__get_log):
   When the start revision is SVN_INVALID_REVNUM, it's always "larger"
   than any valid end revision, so use it for the peg.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568872 13f79535-47bb-0310-9956-ffa450edef68