patch 9.1.0366: filetype: ondir files are not recognized

Problem:  filetype: ondir files are not recognized
Solution: Detect '.ondirrc' as ondir filetype
          (Jon Parise)

closes: #14604

Signed-off-by: Jon Parise <jon@indelible.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/ondir.vim

@@ -0,0 +1,18 @@
+" Vim filetype plugin file
+" Language:     ondir <https://github.com/alecthomas/ondir>
+" Maintainer:   Jon Parise <jon@indelible.org>
+
+if exists('b:did_ftplugin')
+  finish
+endif
+
+let s:cpo_save = &cpoptions
+
+setlocal comments=:# commentstring=#\ %s
+
+let b:undo_ftplugin = 'setl comments< commentstring<'
+
+let &cpoptions = s:cpo_save
+unlet s:cpo_save
+
+" vim: et ts=4 sw=2 sts=2: