The new (1.8) zero_copy_limit parameter in svn_repos_begin_report3
can only be used under certain circumstances.  Document those.

Since we use that interface at our RA layers only, the preconditions
are fulfilled.  However, our tests do some create things with this
API and this patch disables the zero copy optimization for those.

* subversion/include/svn_repos.h
  (svn_repos_begin_report3): mention limitation on zero_copy_limit

* subversion/tests/libsvn_repos/repos-test.c
  (reporter_depth_exclude): make our tests comply to that limitation

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1454789 13f79535-47bb-0310-9956-ffa450edef68