Don't override shell on Windows (#856)

Assume that the user set the shell options correctly
before running vim-plug so that the user can use bash or powershell
as their shell.

Close #815

plug.vim

@@ -794,9 +794,7 @@ endfunction
 
 function! s:chsh(swap)
   let prev = [&shell, &shellcmdflag, &shellredir]
-  if s:is_win
-    set shell=cmd.exe shellcmdflag=/c shellredir=>%s\ 2>&1
-  elseif a:swap
+  if !s:is_win && a:swap
     set shell=sh shellredir=>%s\ 2>&1
   endif
   return prev
@@ -811,7 +809,7 @@ function! s:bang(cmd, ...)
     if s:is_win
       let batchfile = tempname().'.bat'
       call writefile(["@echo off\r", cmd . "\r"], batchfile)
-      let cmd = s:shellesc(batchfile)
+      let cmd = s:shellesc(expand(batchfile))
     endif
     let g:_plug_bang = (s:is_win && has('gui_running') ? 'silent ' : '').'!'.escape(cmd, '#!%')
     execute "normal! :execute g:_plug_bang\<cr>\<cr>"
@@ -1210,7 +1208,7 @@ function! s:spawn(name, cmd, opts)
   let cmd = has_key(a:opts, 'dir') ? s:with_cd(a:cmd, a:opts.dir) : a:cmd
   if !empty(job.batchfile)
     call writefile(["@echo off\r", cmd . "\r"], job.batchfile)
-    let cmd = s:shellesc(job.batchfile)
+    let cmd = s:shellesc(expand(job.batchfile))
   endif
   let argv = add(s:is_win ? ['cmd', '/c'] : ['sh', '-c'], cmd)
 
@@ -2037,7 +2035,7 @@ function! s:system(cmd, ...)
     if s:is_win
       let batchfile = tempname().'.bat'
       call writefile(["@echo off\r", cmd . "\r"], batchfile)
-      let cmd = s:shellesc(batchfile)
+      let cmd = s:shellesc(expand(batchfile))
     endif
     return system(cmd)
   finally
@@ -2371,7 +2369,7 @@ function! s:preview_commit()
     if s:is_win
       let batchfile = tempname().'.bat'
       call writefile(["@echo off\r", cmd . "\r"], batchfile)
-      let cmd = batchfile
+      let cmd = expand(batchfile)
     endif
     execute 'silent %!' cmd
   finally