patch 9.0.1643: filetype detection fails if file name ends in many '~'

Problem:    Filetype detection fails if file name ends in many '~'.
Solution:   Strip multiple '~' at the same time. (closes #12553)

runtime/filetype.vim

@@ -21,7 +21,7 @@ au BufNewFile,BufRead ?\+.orig,?\+.bak,?\+.old,?\+.new,?\+.dpkg-dist,?\+.dpkg-ol
 	\ exe "doau filetypedetect BufRead " . fnameescape(expand("<afile>:r"))
 au BufNewFile,BufRead *~
 	\ let s:name = expand("<afile>") |
-	\ let s:short = substitute(s:name, '\~$', '', '') |
+	\ let s:short = substitute(s:name, '\~\+$', '', '') |
 	\ if s:name != s:short && s:short != "" |
 	\   exe "doau filetypedetect BufRead " . fnameescape(s:short) |
 	\ endif |

src/testdir/test_filetype.vim

@@ -1,5 +1,16 @@
 " Test :setfiletype
 
+func Test_backup_strip()
+  filetype on
+  let fname = 'Xdetect.js~~~~~~~~~~~'
+  call writefile(['one', 'two', 'three'], fname, 'D')
+  exe 'edit ' .. fname
+  call assert_equal('javascript', &filetype)
+
+  bwipe!
+  filetype off
+endfunc
+
 func Test_detection()
   filetype on
   augroup filetypedetect

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1643,
 /**/
     1642,
 /**/