patch 8.0.0649: when opening a help file the filetype is set several times

Problem:    When opening a help file the filetype is set several times.
Solution:   When setting the filetype to the same value from a modeline, don't
            trigger FileType autocommands.  Don't set the filetype to "help"
            when it's already set correctly.
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 59531be..adf1e3b 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -6832,8 +6832,9 @@
     char_u	*rt;
     int		mustfree;
 
-    /* set filetype to "help". */
-    set_option_value((char_u *)"ft", 0L, (char_u *)"help", OPT_LOCAL);
+    /* Set filetype to "help" if still needed. */
+    if (STRCMP(curbuf->b_p_ft, "help") != 0)
+	set_option_value((char_u *)"ft", 0L, (char_u *)"help", OPT_LOCAL);
 
 #ifdef FEAT_SYN_HL
     if (!syntax_present(curwin))