patch 8.0.1666: % argument in ch_log() causes trouble

Problem:    % argument in ch_log() causes trouble.
Solution:   Use string as third argument in internal ch_log(). (Dominique
            Pelle, closes #2784)

src/evalfunc.c

@@ -1993,7 +1993,7 @@ f_ch_log(typval_T *argvars, typval_T *rettv UNUSED)
    if (argvars[1].v_type != VAR_UNKNOWN)
	channel = get_channel_arg(&argvars[1], FALSE, FALSE, 0);

    ch_log(channel, (char *)msg);
    ch_log(channel, "%s", msg);
}

/*
@@ -12927,7 +12927,7 @@ get_callback(typval_T *arg, partial_T **pp)
}

/*
 * Unref/free "callback" and "partial" retured by get_callback().
 * Unref/free "callback" and "partial" returned by get_callback().
 */
    void
free_callback(char_u *callback, partial_T *partial)

src/testdir/test_channel.vim

@@ -1823,3 +1823,15 @@ func Test_list_args()
  call s:test_list_args('print("hello\"world\"")', 'hello"world"', 1)
  call s:test_list_args('print("hello\tworld")', "hello\tworld", 1)
endfunc

" Do this last, it stops any channel log.
func Test_zz_ch_log()
  call ch_logfile('Xlog', 'w')
  call ch_log('hello there')
  call ch_log('%s%s')
  call ch_logfile('')
  let text = readfile('Xlog')
  call assert_match("hello there", text[1])
  call assert_match("%s%s", text[2])
  call delete('Xlog')
endfunc

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    1666,
/**/
    1665,
/**/