Always convert backslashes on Windows

The old conditional was fine for most things, but this ensures that
backslashes are used even for raw user input.

References https://github.com/tpope/vim-fugitive/issues/1079

plugin/fugitive.vim

@@ -205,7 +205,7 @@ function! FugitiveGenerate(...) abort
 endfunction
 
 function! s:Slash(path) abort
-  if exists('+shellslash') && !&shellslash
+  if exists('+shellslash')
     return tr(a:path, '\', '/')
   else
     return a:path