plug: restore FZF_DEFAULT_COMMAND on Windows

fzf uses https://github.com/saracen/walker on Windows since v0.21.0
but it does not work on directory symlinks and junctions.
I use directory junctions a lot.
I prefer a list of files by default over nothing.

junegunn/fzf#1847 (comment)
https://github.com/saracen/walker/blob/324a081bae7e580aa0bf3afe8164acb16634afca/walker.go#L85

shared.vim

@@ -129,6 +129,10 @@ if has('win32')
   " Default prompt is not user-friendly.
   " ConEmu's prompt is garbled in winpty.
   let $PROMPT = '$P$_$G$S'
+
+  " fzf default file walker does not work on directory symlinks and junctions
+  " https://github.com/junegunn/fzf/pull/1847#issuecomment-628960827
+  let $FZF_DEFAULT_COMMAND = 'for /r %P in (*) do @(set "_curfile=%P" & set "_curfile=!_curfile:%__CD__%=!" & echo !_curfile!)'
 endif
 
 " {{{tiny