patch 9.0.1771: regex: combining chars in collections not handled

Problem:  regex: combining chars in collections not handled
Solution: Check for following combining characters for NFA and BT engine

closes: #10459
closes: #10286

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

src/testdir/test_regexp_utf8.vim

@@ -575,5 +575,16 @@ func Test_match_too_complicated()
   set regexpengine=0
 endfunc
 
+func Test_combining_chars_in_collection()
+  new
+  for i in range(0,2)
+    exe "set re=".i
+    put =['ɔ̃', 'ɔ',  '̃  ã', 'abcd']
+    :%s/[ɔ̃]//
+    call assert_equal(['', '', 'ɔ', '̃  ã', 'abcd'], getline(1,'$'))
+    %d
+  endfor
+  bw!
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab