patch 8.1.0059: displayed digraph for "ga" wrong with 'encoding' "cp1251"

Problem:    Displayed digraph for "ga" wrong with 'encoding' "cp1251".
Solution:   Convert from 'encoding' to "utf-8" if needed. (closes #3015)

src/digraph.c

@@ -1979,14 +1979,37 @@ do_digraph(int c)
  * If not found return NULL.
  */
     char_u *
-get_digraph_for_char(val)
+get_digraph_for_char(int val_arg)
-    int val;
 {
+    int		val = val_arg;
     int		i;
     int		use_defaults;
     digr_T	*dp;
     static      char_u      r[3];
 
+#if defined(FEAT_MBYTE) && defined(USE_UNICODE_DIGRAPHS)
+    if (!enc_utf8)
+    {
+	char_u	    buf[6], *to;
+	vimconv_T   vc;
+
+	// convert the character from 'encoding' to Unicode
+	i = mb_char2bytes(val, buf);
+	vc.vc_type = CONV_NONE;
+	if (convert_setup(&vc, p_enc, (char_u *)"utf-8") == OK)
+	{
+	    vc.vc_fail = TRUE;
+	    to = string_convert(&vc, buf, &i);
+	    if (to != NULL)
+	    {
+		val = utf_ptr2char(to);
+		vim_free(to);
+	    }
+	    (void)convert_setup(&vc, NULL, NULL);
+	}
+    }
+#endif
+
     for (use_defaults = 0; use_defaults <= 1; use_defaults++)
     {
 	if (use_defaults == 0)

src/testdir/test_digraph.vim

@@ -465,4 +465,16 @@ func Test_show_digraph()
   bwipe!
 endfunc
 
+func Test_show_digraph_cp1251()
+  if !has('multi_byte')
+    return
+  endif
+  new
+  set encoding=cp1251
+  call Put_Dig("='")
+  call assert_equal("\n<\xfa>  <|z>  <M-z>  250,  Hex fa,  Oct 372, Digr ='", execute('ascii'))
+  set encoding=utf-8
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    59,
 /**/
     58,
 /**/