patch 9.1.1006: PmenuMatch completion highlight can be combined

Problem:  PmenuMatch completion highlight can be combined
Solution: Combine highlight groups PmenuMatch with Pmenu and
          PmenuMatchSel with PmenuSel (glepnir)

fixes: #15563
closes: #16408

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/popupmenu.c b/src/popupmenu.c
index ea2edca..14cc69a 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -450,13 +450,18 @@
 		if (char_pos == ((int_u *)ga->ga_data)[i])
 		{
 		    new_attr = highlight_attr[is_select ? HLF_PMSI : HLF_PMNI];
+		    new_attr = hl_combine_attr(highlight_attr[hlf], new_attr);
 		    break;
 		}
 	    }
 	}
 	else if (matched_start && ptr < text + leader_len)
+	{
 	    new_attr = highlight_attr[is_select ? HLF_PMSI : HLF_PMNI];
+	    new_attr = hl_combine_attr(highlight_attr[hlf], new_attr);
+	}
 
+	new_attr = hl_combine_attr(highlight_attr[HLF_PNI], new_attr);
 	if (user_hlattr > 0)
 	    new_attr = hl_combine_attr(new_attr, user_hlattr);