patch 8.0.0650: for extra help files the filetype is set more than once

Problem:    For extra help files the filetype is set more than once.
Solution:   In *.txt files check that there is no help file modline.

runtime/filetype.vim

@@ -1,7 +1,7 @@
 " Vim support file to detect file types
 "
 " Maintainer:	Bram Moolenaar <Bram@vim.org>
-" Last Change:	2017 Jun 20
+" Last Change:	2017 Jun 22
 
 " Listen very carefully, I will say this only once
 if exists("did_load_filetypes")
@@ -2789,7 +2789,13 @@ au BufNewFile,BufRead zsh*,zlog*		call s:StarSetf('zsh')
 
 " Plain text files, needs to be far down to not override others.  This avoids
 " the "conf" type being used if there is a line starting with '#'.
-au BufNewFile,BufRead *.txt,*.text,README	setf text
+au BufNewFile,BufRead *.text,README		setf text
+
+" Help files match *.txt but should have a last line that is a modeline.
+au BufNewFile,BufRead *.txt	
+	\  if getline('$') !~ 'vim:.*ft=help'
+	\|   setf text
+	\| endif
 
 
 " Use the filetype detect plugins.  They may overrule any of the previously

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    650,
 /**/
     649,
 /**/