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,
 /**/