Remove magic string length constants and a duplicative function.

* subversion/libsvn_ra_svn/marshal.c
  (writebuf_write_short_string): Delete. This function was a wrapper around
    writebuf_write that essentially repeated the same optimization.
  (writebuf_write_literal): New macro.
  (svn_ra_svn__write_word): Call writebuf_write instead of
    writebuf_write_short_string.
  (...everywhere else...): Use writebuf_write_literal instead of passing a
    magic string length constant to writebuf_write_short_string.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1587498 13f79535-47bb-0310-9956-ffa450edef68