Eliminate an unnecessary OPTIONS request in HTTPv1 communications.
(Not only is it unnecessary, but sometimes it's aimed at an unreadable
URL, causing commits to fail unnecessarily.)

* subversion/libsvn_ra_serf/commit.c
  (open_root): Only call svn_ra_serf__v1_get_activity_collection() if
    we haven't already cached an activity collection URL.  (And cache
    its result if do have to call it.)

* subversion/libsvn_ra_serf/options.c
  (svn_ra_serf__exchange_capabilities): Cache the activity collection
    URL if one is reported.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__session_t): Add 'activity_collection_url' member.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1454088 13f79535-47bb-0310-9956-ffa450edef68