patch 8.2.1673: complete_info() selected index has an invalid value

Problem:    complete_info() selected index has an invalid value. (Ben Jackson)
Solution:   Set the index when there is only one match. (closes #6945)
            Add test for complete_info().

src/insexpand.c

@@ -4009,6 +4009,7 @@ ins_complete(int c, int enable_pum)
 	{
 	    edit_submode_extra = (char_u *)_("The only match");
 	    edit_submode_highl = HLF_COUNT;
+	    compl_curr_match->cp_number = 0;
 	}
 	else
 	{

src/testdir/test_ins_complete.vim

@@ -313,6 +313,24 @@ func Test_CompleteDone_undo()
   au! CompleteDone
 endfunc
 
+func CompleteTest(findstart, query)
+  if a:findstart
+    return col('.')
+  endif
+  return ['matched']
+endfunc
+
+func Test_completefunc_info()
+  new
+  set completeopt=menuone
+  set completefunc=CompleteTest
+  call feedkeys("i\<C-X>\<C-U>\<C-R>\<C-R>=string(complete_info())\<CR>\<ESC>", "tx")
+  call assert_equal("matched{'pum_visible': 1, 'mode': 'function', 'selected': -1, 'items': [{'word': 'matched', 'menu': '', 'user_data': '', 'info': '', 'kind': '', 'abbr': ''}]}", getline(1))
+  bwipe!
+  set completeopt&
+  set completefunc&
+endfunc
+
 " Check that when using feedkeys() typeahead does not interrupt searching for
 " completions.
 func Test_compl_feedkeys()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1673,
 /**/
     1672,
 /**/