Tweak ra_serf report xml generation code a tiny bit to use the shorter
'<element />' notation for elements that are always just markers for
enabling a feature instead of '<element></element>'.

This simplifies and shortens the requests a tiny bit and returns a few of
those requests to their easier to process neon form.

* subversion/libsvn_ra_serf/blame.c
  (create_file_revs_body): Use short form for merge info flag.

* subversion/libsvn_ra_serf/commit.c
  (create_checkout_body): Use short form for element.

* subversion/libsvn_ra_serf/get_lock.c
  (create_getlock_body): Use short form for element.

* subversion/libsvn_ra_serf/lock.c
  (create_lock_body): Use short form for two elements.

* subversion/libsvn_ra_serf/log.c
  (create_log_body): Use short form for quite a few elements.

* subversion/libsvn_ra_serf/merge.c
  (create_merge_body): Use short form for quite a few elements.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__add_empty_tag_buckets): New function.

* subversion/libsvn_ra_serf/xml.c
  (svn_ra_serf__add_empty_tag_buckets): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1627333 13f79535-47bb-0310-9956-ffa450edef68