Check shell in Windows before assuming cmd.exe

In Windows, some users set the Cygwin shell as their Vim shell, make
Vundle consider this when deciding which flavour of commands to run when
cloning. Roughly check if the shell contains `sh` in it, and assume a
Unix shell, even when on Windows.

autoload/vundle/installer.vim

@@ -169,7 +169,7 @@ endf
 
 func! vundle#installer#delete(bang, dir_name) abort
 
-  let cmd = (has('win32') || has('win64')) ?
+  let cmd = ((has('win32') || has('win64')) && empty(matchstr(&shell, 'sh'))) ?
   \           'rmdir /S /Q' :
   \           'rm -rf'
 
@@ -251,7 +251,7 @@ func! s:sync(bang, bundle) abort
 endf
 
 func! g:shellesc(cmd) abort
-  if (has('win32') || has('win64'))
+  if ((has('win32') || has('win64')) && empty(matchstr(&shell, 'sh')))
     if &shellxquote != '('                           " workaround for patch #445
       return '"'.a:cmd.'"'                          " enclose in quotes so && joined cmds work
     endif
@@ -260,7 +260,7 @@ func! g:shellesc(cmd) abort
 endf
 
 func! g:shellesc_cd(cmd) abort
-  if (has('win32') || has('win64'))
+  if ((has('win32') || has('win64')) && empty(matchstr(&shell, 'sh')))
     let cmd = substitute(a:cmd, '^cd ','cd /d ','')  " add /d switch to change drives
     let cmd = g:shellesc(cmd)
     return cmd