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/version.c b/src/version.c
index b221ce2..47e1da5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    649,
+/**/
     648,
 /**/
     647,