Add a tiny bit of code to allow testing with Apache Serf's http/2 support.
Add a tiny bit of code to allow testing with Apache Serf's http/2 support.

I committed this patch to celebrate that I got through basic_tests.py
using the current http/2 support.

* subversion/libsvn_ra_serf/util.c
  (conn_negotiate_protocol): New function.
  (conn_setup): Register usage of conn_negotiate_protocol when
    a very recent (read: trunk) serf + SVN__SERF_TEST_HTTP2 are used.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1715228 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion