patch 9.0.0935: when using dash it may not be recognize as filetype "sh"

Problem:    When using dash it may not be recognize as filetype "sh".
Solution:   Add checks for "dash". (Eisuke Kawashima,closes #11600)
diff --git a/runtime/autoload/dist/ft.vim b/runtime/autoload/dist/ft.vim
index 8f74445..c40e0c8 100644
--- a/runtime/autoload/dist/ft.vim
+++ b/runtime/autoload/dist/ft.vim
@@ -712,7 +712,8 @@
     if exists("b:is_sh")
       unlet b:is_sh
     endif
-  elseif name =~ '\<sh\>'
+  elseif name =~ '\<sh\>' || name =~ '\<dash\>'
+    # Ubuntu links "sh" to "dash", thus it is expected to work the same way
     b:is_sh = 1
     if exists("b:is_kornshell")
       unlet b:is_kornshell