Revision 1663500

In ra-serf (for issue #4557) reinstate a bit of code that retries a delete
with an altered request if the original request fails, because the server
determined that it is an invalid request, because it has too many bytes
in the headers.

Before r1553501 we always retried DELETE requests that required lock
tokens, as the initial request didn't have an If header at all.

* subversion/libsvn_ra_serf/commit.c
  (delete_context_t): Add boolean.
  (setup_delete_headers): Only setup a single If when recursive lock
    headers are disabled.
  (create_delete_body): New function, based on the version removed in
    r1553501.
  (delete_entry): Detect a failed request and run a retry if necessary.
  (add_directory): Document limitation.

* subversion/libsvn_ra_serf/merge.c
  (merge_lock_token_list): Rename back to...
  (svn_ra_serf__merge_lock_token_list): ... this.

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

* subversion/tests/cmdline/lock_tests.py
  (delete_dir_with_lots_of_locked_files): Extend test to show
    that the MKCOL case isn't fixed.