Revision 1704891

Following up on r1704317, advertise svndiff1 support in mod_dav_svn servers.

When parsing OPTIONS responses in ra_serf, record that we can use svndiff1
upon seeing the new DAV header.  If there is no such header, use the current
approach of sending svndiff1 to servers that support Subversion's HTTPv2
protocol.

Related discussion is in http://svn.haxx.se/dev/archive-2015-09/0278.shtml
(Subject: "Re: svn commit: r1704317 - ...").

* subversion/include/svn_dav.h
  (SVN_DAV_NS_DAV_SVN_SVNDIFF1): New.

* subversion/mod_dav_svn/version.c
  (get_vsn_options): Advertise that svndiff1 is supported by the server.

* subversion/libsvn_ra_serf/options.c
  (capabilities_headers_iterator_callback): Record svndiff1 support if the
   server advertises it with the new header.  Update the related comment.

Suggested by: danielsh