Report repository merge tracking capability using HTTPv2 like header 
'SVN-Repository-MergeInfo'.

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

* subversion/libsvn_ra_serf/options.c
  (capabilities_headers_iterator_callback): Set SVN_RA_CAPABILITY_MERGEINFO
   to CAPABILITY_SERVER_YES only if didn't receive explicit repository merge
   tracking capability. Handle new SVN_DAV_REPOSITORY_MERGEINFO header.
  (options_response_handler): Reset SVN_RA_CAPABILITY_MERGEINFO to NULL 
   before iterating headers. 

* subversion/mod_dav_svn/version.c
  (get_option): Report repository merge tracking capability using 
   SVN_DAV_REPOSITORY_MERGEINFO header.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1445083 13f79535-47bb-0310-9956-ffa450edef68