Revision 1801974

Negotiate the use of the svndiff2 format over http://.

On the server-side, start sending svndiff2 deltas to clients that can read
them, but only when SVNCompressionLevel is set to 1.  On the client-side,
keep sending svndiff1.  Currently we use svndiff1 with default compression
ratio (5), and svndiff2 is not a substitute for it, as that would result
in worse compression.

* subversion/include/svn_dav.h
  (SVN_DAV_NS_DAV_SVN_SVNDIFF2): New.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__session_t): Add 'supports_svndiff2' field.

* subversion/libsvn_ra_serf/options.c
  (capabilities_headers_iterator_callback): Remember if the server supports
   svndiff2.

* subversion/libsvn_ra_serf/commit.c
  (apply_textdelta): Add the fallback code for a theoretically possible
   situation where the server has advertised _only_ svndiff2 support.
   Note why we still use svndiff1 when client-side http compression is
   enabled.  Adjust a couple of comments.

* subversion/libsvn_ra_serf/util.c
  (svn_ra_serf__setup_svndiff_accept_encoding): Advertise support for both
   svndiff2 and svndiff1 with identical (q=) values.  We'll let the server
   decide between svndiff2 and svndiff1.

* subversion/mod_dav_svn/repos.c
  (negotiate_encoding_prefs): Select the svndiff2 format if the client
   supports it and if the server's SVNCompressionLevel is set to 1.

* subversion/mod_dav_svn/version.c
  (get_vsn_options): Advertise svndiff2 support.