Move the get_file() implementation in ra_serf to a file of its own. It doesn't
share a relevant enough bit of code with the update editor to allow peaking in
its structures.

* subversion/libsvn_ra_serf/get_file.c
  New file copied from update.c.
  (fetch_ctx_t): Kept as stream_ctx_t with some removals.
  (headers_fetch): Kept for the 50% relevant for this.
  (cancel_fetch): Kept.
  (try_get_wc_contents,
   handle_stream,
   svn_ra_serf__get_file): Kept.

* subversion/libsvn_ra_serf/update.c
  (try_get_wc_contents,
   handle_stream,
   svn_ra_serf__get_file): Remove here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557565 13f79535-47bb-0310-9956-ffa450edef68