Fix check for need to convert Windows path slashes

Closes 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 &shell =~? 'cmd' || exists('+shellslash') && !&shellslash
+  if exists('+shellslash') && !&shellslash
     return tr(a:path, '\', '/')
   else
     return a:path