patch 8.1.1228: not possible to process tags with a function

Problem:    Not possible to process tags with a function.
Solution:   Add tagfunc() (Christian Brabandt, Andy Massimino, closes #4010)
diff --git a/src/insexpand.c b/src/insexpand.c
index ad95acc..eeff64c 100644
--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -2654,11 +2654,13 @@
 
 	    // Find up to TAG_MANY matches.  Avoids that an enormous number
 	    // of matches is found when compl_pattern is empty
+	    g_tag_at_cursor = TRUE;
 	    if (find_tags(compl_pattern, &num_matches, &matches,
 		    TAG_REGEXP | TAG_NAMES | TAG_NOIC | TAG_INS_COMP
 		    | (ctrl_x_mode != CTRL_X_NORMAL ? TAG_VERBOSE : 0),
 		    TAG_MANY, curbuf->b_ffname) == OK && num_matches > 0)
 		ins_compl_add_matches(num_matches, matches, p_ic);
+	    g_tag_at_cursor = FALSE;
 	    p_ic = save_p_ic;
 	    break;