patch 9.1.0666: assert_equal() doesn't show multibyte string correctly

Problem:  assert_equal() doesn't show multibyte string correctly
Solution: Properly advance over a multibyte char.
          (zeertzjq)

closes: #15456

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_assert.vim

@@ -48,10 +48,19 @@ func Test_assert_equal()
   call assert_match("Expected 'bar' but got 'foo'", v:errors[0])
   call remove(v:errors, 0)
 
+  let s = 'αβγ'
+  call assert_equal(1, assert_equal('δεζ', s))
+  call assert_match("Expected 'δεζ' but got 'αβγ'", v:errors[0])
+  call remove(v:errors, 0)
+
   call assert_equal('XxxxxxxxxxxxxxxxxxxxxxX', 'XyyyyyyyyyyyyyyyyyyyyyyyyyX')
   call assert_match("Expected 'X\\\\\\[x occurs 21 times]X' but got 'X\\\\\\[y occurs 25 times]X'", v:errors[0])
   call remove(v:errors, 0)
 
+  call assert_equal('ΩωωωωωωωωωωωωωωωωωωωωωΩ', 'ΩψψψψψψψψψψψψψψψψψψψψψψψψψΩ')
+  call assert_match("Expected 'Ω\\\\\\[ω occurs 21 times]Ω' but got 'Ω\\\\\\[ψ occurs 25 times]Ω'", v:errors[0])
+  call remove(v:errors, 0)
+
   " special characters are escaped
   call assert_equal("\b\e\f\n\t\r\\\x01\x7f", 'x')
   call assert_match('Expected ''\\b\\e\\f\\n\\t\\r\\\\\\x01\\x7f'' but got ''x''', v:errors[0])

src/testing.c

@@ -99,7 +99,7 @@ ga_concat_shorten_esc(garray_T *gap, char_u *str)
 	return;
     }
 
-    for (p = str; *p != NUL; ++p)
+    for (p = str; *p != NUL; )
     {
 	same_len = 1;
 	s = p;
@@ -118,10 +118,13 @@ ga_concat_shorten_esc(garray_T *gap, char_u *str)
 	    vim_snprintf((char *)buf, NUMBUFLEN, "%d", same_len);
 	    ga_concat(gap, buf);
 	    ga_concat(gap, (char_u *)" times]");
-	    p = s - 1;
+	    p = s;
 	}
 	else
+	{
 	    ga_concat_esc(gap, p, clen);
+	    p += clen;
+	}
     }
 }
 

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    666,
 /**/
     665,
 /**/