Revision 1803141

ra_serf: Add new bucket that wraps a readable svn_stream_t.

This new bucket would be required to make ra_serf stream svndiff deltas
without creating temporary files.  (We'll take an svn_txdelta_stream_t,
convert it to a generic svn_stream_t with svn_txdelta_to_svndiff_stream(),
create a bucket around that stream, and use it as the PUT request body.
This change introduces adds the necessary bucket implementation.)

* subversion/include/svn_error_codes.h
  (SVN_ERR_RA_SERF_STREAM_BUCKET_READ_ERROR): New.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__stream_bucket_errfunc_t): Declare new callback type.
  (svn_ra_serf__create_stream_bucket): Declare new function.

* subversion/libsvn_ra_serf/stream_bucket.c: New file.
  (stream_bucket_ctx_t): New.
  (svn_ra_serf__create_stream_bucket): New, creates the bucket and
   initializes a databuf that would be used in its implementation.
   Remember the passed-in errfunc callback to allow reporting extended
   error information to the caller in the form of an svn_error_t.
  (stream_bucket_read, stream_bucket_readline, stream_bucket_peek):
   New bucket vtable members that forward everything to the databuf.
  (stream_reader): New.  This is the core function of the new bucket that
   reads data from the svn_stream_t and possibly forwards the errors
   using the stored errfunc callback.