When not using send-all in ra-serf, report files as opened a bit earlier.
This allows using the txdelta handler pointer as a way to signal that we
don't need the file request at all.

This avoids transferring files in several diff and merge scenarios, and also
when update has to skip trees for certain kinds of obstructions.

* subversion/libsvn_ra_serf/update.c
  (handle_fetch): Don't report the file open here
  (handle_local_content): ... or here.

  (fetch_file): But do it a bit earlier in here, where we can use
    the handler pointer to avoid doing any work at all.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1449686 13f79535-47bb-0310-9956-ffa450edef68